STRUCTURAL  AND 
FIELD    GEOLOGY, 

FOR   STUDENTS   OF    PURE   AND 
APPLIED    SCIENCE 


BY 

JAMES  GEIKIE,  LL.D.,  D.C.L.,  F.E.S.,  ETC. 

MURCHISON   PROFESSOR  OF  GEOLOGY  AND  MINERALOGY  IN  THE   UNIVERSITY  OF  EDINBURGH 
FORMERLY   OF  H.M.   GEOLOGICAL  SURVEY 

AUTHOR  OF 
"THE  GRKAT  ICE  AGE,"  "PREHISTORIC  EUROPE,"  "EARTH  SCULPTURE,"  ETC. 


SECOND    EDITION,    REVISED 


NEW    YOBK 
D.   VAN    NOSTRAND    COMPANY 

23  MURRAY  AND  27  WARREN  STREETS 
1910 


PREFACE  TO  THE  FIRST  EDITION 


THIS  Handbook  addresses  itself,  in  the  first  place,  to  beginners 
in  Field  Geology,  but  I  hope  it  may  be  found  useful  also  to 
students  who  are  preparing  for  professions  in  which  some 
knowledge  of  Structural  Geology  is  of  practical  importance. 
The  amount  of  geological  training  demanded  varies,  doubt- 
less, with  the  nature  of  the  profession.  Mining  engineers, 
for  example,  must  acquire  a  knowledge  of  many  details 
which  civil  engineers,  architects,  agriculturists,  and  public 
health  officers  can  afford  to  neglect.  Nevertheless,  if 
Structural  Geology  is  to  be  of  service  to  a  professional  man 
it  must  be  studied  in  a  systematic  manner.  Without  an 
intelligent  appreciation  of  the  subject  as  a  whole,  it  is  very 
hard  or  well-nigh  impossible  to  gain  an  adequate  working 
knowledge  of  any  particular  part.  In  the  following  pages, 
therefore,  the  subject  is  set  forth  mainly  from  the  point  of 
view  of  pure  science.  The  student  of  applied  science, 
however,  should  have  little  difficulty  in  distinguishing  between 
matter  of  general  interest,  and  that  which  is  of  special 
importance  to  him,  as  bearing  directly  on  his  own  professional 
pursuits.  To  help  in  this  discrimination  two  sizes  of  type 
have  been  employed— the  smaller  type  being  commonly 
reserved  for  details  or  discussions  of  import  mainly  or 
exclusively  to  students  of  pure  science.  With  regard  to  the 
matter  in  larger  type,  the  intelligent  student  will  use  his  own 
discretion.  To  others  than  mining  men,  for  example,  the 
chapters  dealing  with  ore-formations  will  not  call  for  much 
studious  consideration.  Again,  neither  civil  engineers,  public 
health  officers,  nor  agriculturists  may  ever  be  called  upon  to 

236399 


vi  PREFACE  TO  THE  FIRST  EDITION 

make  a  geological  survey  of  any  district.  Intending  pro- 
fessional men  will  be  ill-advised,  however,  if  they  do  not  take 
the  trouble  to  understand  the  methods  of  observation 
employed  in  Field  Geology,  for  such  knowledge  will  often  be 
of  considerable  service  in  their  fyture  careers.  To  mining 
and  civil  engineers,  especially,  an  acquaintance  with  the 
methods  of  geological  surveying  and  map-construction  cannot 
fail  to  be  invaluable,  while  agriculturists  and  public  health 
officers  ought  assuredly  to  know  enough  of  the  subject  to 
understand  and  interpret  a  geological  map.  I  may  be 
allowed  to  add  that  at  the  University  of  Edinburgh  we  have 
found  no  difficulty  in  teaching  Structural  and  Field  Geology 
to  mixed  classes  of  students  of  pure  and  applied  science. 
The  present  Manual  may  be  said  to  cover  the  ground  gone 
over  in  our  Summer  Course  of  Geology — a  course  instituted 
some  twenty  years  ago  to  meet  the  requirements  of  students 
desirous  of  obtaining  a  fuller  knowledge  of  Practical  Geology 
— more  especially  field-work — than  could  be  presented  in  the 
general  systematic  course  given  in  winter. 

The  Plates  which  illustrate  this  volume  have  been  derived 
from  various  sources.  A  number  are  reproduced  from 
unpublished  photographs  taken  by  Mr  R.  Lunn  for  H.M. 
Geological  Survey.  Permission  to  use  these  was  obtained 
from  the  Board  of  Education  through  the  kind  offices  of  Dr 
Teall,  Director  of  the  Geological  Survey,  and  Dr  Home, 
Assistant  Director.  No  one  can  be  more  sensible  than 
myself  that  these  illustrations  give  an  interest  to  this  work 
which  it  would  not  otherwise  possess.  To  my  former 
colleague  and  lifelong  friend,  Dr  Peach,  I  am  indebted  for  the 
coloured  section  which  accompanies  one  of  these  plates. 
Plates  X.  and  XXIV.  are  reproduced,  by  the  courteous  permis- 
sion of  the  Controller  of  H.M.  Stationery  Office,  from  published 
memoirs  of  the  Geological  Survey.  My  friend  and  former 
assistant,  Dr  Flett,  now  of  H.M.  Geological  Survey,  was  good 
enough  to  supply  me  with  the  photograph  reproduced  on 
Plate  XXX  I.,  as  well  as  with  others  which  only  the  limited  scope 
of  my  book  has  prevented  me  using.  I  am  also  under  many 
obligations  to  him  for  reading  some  of  my  proof-sheets,  and 
making  various  helpful  suggestions.  To  another  friend  and 
former  pupil,  Dr  Laurie,  I  am  similarly  indebted  for  the 


PREFACE  TO  THE  FIRST  EDITION  vii 

photographs  reproduced  on  Plates  XXV.  and  L IV.,  which  were 
taken  on  one  of  the  excursions  of  my  Summer  Class.  Mr 
Francis  J.  Lewis,  of  the  University  of  Liverpool,  whose 
investigations  into  the  structure  and  history  of  the  peat-bogs 
of  Britain  promise  to  be  of  the  greatest  interest  and  import- 
ance to  botanists  and  geologists,  kindly  put  at  my  disposal 
several  characteristic  photographs  of  peat-bogs,  from  which  I 
selected  the  illustration  that  appears  in  Plate  LIV.  Unless 
when  otherwise  stated,  all  the  other  Plates  are  reproductions 
from  photographs  of  specimens  in  my  own  Class-Museum, 
taken  under  the  superintendence  of  Dr  J.  D.  Falconer,  formerly 
my  assistant  and  now  Director  of  the  Mineral  Survey  of 
Northern  Nigeria.  I  may  add  that  many  of  the  illustrations 
in  the  text  were  drawn  for  me  by  my  son,  Mr  W.  Cranston 
Geikie. 


EDINBURGH,  April  15,  1905. 


PREFACE  TO  THE  SECOND  EDITION 


THIS  edition  differs  but  little  from  its  predecessor.  The 
author  has  taken  the  opportunity  it  has  afforded  him,  however, 
to  supply  some  omissions,  and  to  make  a  number  of  emenda- 
tions and  corrections,  which  he  hopes  may  render  the  work 
more  acceptable  to  those  for  whom  it  has  been  prepared. 


EDINPURGH,  February  24,  1908, 


CONTENTS 

CHAPTER  I 

ROCK-FORMING   MINERALS 

PAGE 

Oxides  : — Quartz  and  its  varieties  ;  Opal ;  Specular  Iron  ;  Ilmenite  ; 
Magnetite  ;  Limonite  ;  Rutile  ;  Zircon  ;  Spinelloids  ;  Corun- 
dum ;  Pyrolusite,  Psilomelane,  and  Wad.  Silicates  : — Felspar 
Group ;  Felspathoid  Group  .  .  .  .  .  1-15 

CHAPTER  II 

ROCK-FORMING   MINERALS — continued 

Silicates  : — Amphibole  and  Pyroxene  Group  ;  Mica  Group  ;  Olivine 
Group ;  Chlorite  Group ;  Talc  Group ;  Epidote  Group ; 
Garnet  Group  ;  Tourmaline  Group  ;  Titanite  Group  ;  Anda- 
lusite  Group  ;  Zeolite  Group  ;  Kaolinite  Group.  Haloids — 
Fluorite  and  Rock-Salt.  Sulphides — Pyrite,  Pyrrhotite,  and 
Marcasite.  Carbonates — Calcite,  Aragonite,  Dolomite,  and 
Siderite*  Sulphates  —  Anhydrite,  Gypsum,  and  Barytes. 
Phosphates — Apatite,  etc.  Elements — Graphite  .  .  16-31 

CHAPTER  III 

ROCKS 

Classification  : —  Crystalline  Igneous  Rocks  —  their  General 
Characters.  Chief  Minerals  of  Igneous  Rocks.  Primary  and 
Secondary  Minerals.  Law  of  Mineral  Combination.  Groups 
of  Igneous  Rocks  : — Rocks  with  Dominant  Alkali  Felspar  ; 
Rocks  with  Dominant  Soda-Lime  Felspar ;  Rocks  with 
Felspathoids  in  place  of  Felspars  ;  Rocks  without  Felspars 
or  Felspathoids  ;  Pyroclastic  Rocks  ....  32-54 

CHAPTER  IV 

ROCKS — continued 

Classification  of  Derivative  Rocks  : — I.  Mechanically  formed 
Rocks,  including  Subaerial  and  ^Eolian,  Sedimentary,  and 
Glacial  Rocks  (Soil  and  Subsoil,  Rock-rubble,  Rain-wash, 
etc.,  Blown  Sand  and  Dust,  Laterite,  Terra  Rossa,  Conglome- 
rate, Grit  and  Sandstone,  Greywacke,  Clay,  Till,  etc.).  II. 
Chemically  formed  Rocks— (Stalactites  and  Stalagmites,  Tufa, 
Magnesian  Limestone,  Rock-salt,  Gypsum,  Siliceous  Sinter, 
Flint,  etc.,  Ironstones).  III.  Organically  derived  Rocks — 
(Limestone,  Coal.,  etc,  Guano,  Coprolites)  .  .  .  55-73 

1X  a  2 


x  CONTENTS 

CHAPTER  V 

ROCKS — continued 

PAGK 

Metamorphic  Rocks  :— A.  Schistose  Rocks— their  General  Char- 
acters. Quartzose  Rocks.  Argillaceous  Rocks.  Mica-schist. 
Gneiss.  Chlorite-schist.  Talc-schist.  Amphibolites.  Granu- 
lite.  Marble.  Serpentine.  B.  Cataclastic  Rocks — their 
General  Characters.  Mylonites.  Friction-breccias.  Deter- 
mination of  Rocks  in  the  Field.  General  Characters  of 
Argillaceous,  Calcareous,  Siliceous,  and  Felspathic  Rocks. 
Specific  Gravity  of  Rocks  .....  74-89 

CHAPTER  VI 

FOSSILS 

Modes  of  Preservation  of  Organic  Remains.  Kinds  of  Rock  in 
which  Fossils  occur.  Fossils  chiefly  of  Marine  Origin.  Im- 
portance of  Fossils  in  Geological  Investigations.  Climatic 
and  Geographical  Conditions  and  Terrestrial  Movements 
deduced  from  Fossils.  Geological  Chronology  and  Fossils  90-103 

CHAPTER  VII 
STRATIFICATION   AND  THE   FORMATION    OF   ROCK-BEDS 

Consolidation  of  Incoherent  Accumulations.  Lamination  and 
Stratification.  Extent  and  Termination  of  Beds.  Contem- 
poraneous Erosion.  Grouping  of  Strata.  Contemporaneity 
of  Strongly  Contrasted  Strata.  Diagonal  Lamination  and 
Stratification.  Surface  markings  .  .  .  104-119 

CHAPTER  VIII 

CONCRETIONARY  AND  SECRETIONARY  STRUCTURES 

Siliceous  Concretions — Flint,  Chert,  Menilite.  Calcareous  and 
Ferruginous  Concretions  —  Septaria,  Composite  Nodules, 
Rattle-stones,  Fairy-stones,  Kankar,  etc.  Clay-ironstone 
Nodules,  Pyrite,  Marcasite,  Gypsum,  Dendrites.  Concretion- 
ary Sandstones,  Argillaceous  Rocks,  and  Limestones.  Con- 
cretionary Tuffs.  Concretions  in  Crystalline  Igneous  Rocks. 
Secretionary  Structures — Amygdules,  Geodes,  Drusy  Cavities  120-127 

CHAPTER  IX 

INCLINATION   AND   CURVATURE  OF  STRATA 

Dip — Apparent  and  True.  Terminal  Curvature.  Outcrop  in- 
fluenced by  Angle  of  Dip  and  Form  of  Ground.  Strike. 
Curvature  of  Strata — Monoclinal  Folds,  Quaquaversal  and 
Centrodinal  Folds,  Normal  or  Symmetrical  Folds,  Unsym- 
metrical  Folds,  Inversion,  Recumbent  Folds,  Fan-shaped 
Structure,  Contorted  Strata.  Origin  of  Folds  .  .  127-143 

v 


CONTENTS 

CHAPTER  X 

JOINTS 

Joints  Close  and  Gaping.  Joints  in  Bedded  Rocks — Master-joints, 
Dip-  and  Strike-joints.  Joints  in  Igneous  Rocks — in  Granitoid 
Rocks,  Prismatic  Joints.  Joints  in  Schistose  Rocks.  Slicken- 
sides.  Origin  of  Joints — Contraction,  Expansion,  Crustal 
Movements  .  .  .  .  .  •  1 44-154 

CHAPTER  XI 
FAULTS  OR  DISLOCATIONS 

Normal  Faults.  Dip-faults  and  Strike-faults— their  Effect  upon 
Outcrops.  Oblique  Faults.  Systems  of  Faults.  Step-faults. 
Trough-  and  Ridge-faults.  Shifting  of  Faults.  Reversed 
Faults.  Transcurrent  Faults.  Origin  of  Faults  .  1 55-^76 


CHAPTER  XII 

STRUCTURES  RESULTING  FROM  DENUDATION 
Outliers  and  Inliers.     Unconformity.     Overlap        .  .  177-183 

CHAPTER   XIII 

ERUPTIVE   ROCKS  :   MODE   OF   THEIR  OCCURRENCE 

Intrusive  Eruptive  Rocks.  Plutonic  or  Abyssal  and  Hypabyssal 
Rocks — their  General  Petrographical  Characters.  Batholiths 
— Granite  as  a  type  ;  phenomena  along  line  of  Junction  with 
Contiguous  Rocks  ;  Xenoliths  ;  speculations  as  to  Assimilation 
of  Rocks  by  Granite,  etc.  Laccoliths  of  North  America. 
Sills  or  Intrusive  Sheets  appear  to  be  much  denuded  Lacco- 
liths. Necks  or  Pipes  of  Eruption — their  General  Phenomena  184-201 

CHAPTER   XIV 

ERUPTIVE  ROCKS:   MODE  OF  THEIR  OCCURRENCE — 
continued 

Dykes  and  Eruptive  Veins — their  General  Phenomena.  Composite 
Dykes.  Exogenous  or  Intrusive  Veins — their  association  with 
Batholithsj  etc.  Endogenous  or  Autogenous  Veins — Pegmatite 
Veins ;  General  Phenomena  of  ^Dniasmporaneous  3£ems. 
Segregation  Veins.  Effusive  Eruptive  Hocks — Crystalline 
Effusive  Rocks  and  Pyroclastic  or  Fragmental  Effusive 
Rocks  .  202-211 


xii  CONTENTS 

CHAPTER    XV 

ALTERATION   AND   METAMORPHISM 

PAGE 

Rock-changes  induced  by  Epigene  Action.  Deep-seated  Alteration 
or  Metamorphism.  Degrees  of  Metamorphism.  Thermal 
or  Contact  Metamorphism.  Regional  Metamorphism — 
Plutonic,  Hydro-chemical,  and  Dynamo-metamorphism  212-227 

CHAPTER  XVI 

ORE-FORMATIONS 

Syngenetic  Ore-Formations — Native  Metals  and  Ores  in  Igneous 
Rocks  ;  Ores  in  Bedded  Rocks  (Chemical  Precipitates, 
Clastic  Ores,  Ores  in  Schists).  Epigenetic  Ore-Formations — 
Fissure  Veins  or  Lodes ;  Nature  of  Fissures ;  Width  and 
Extent  of  Lodes  ;  Simple  and  Complex  Lodes  ;  Transverse 
and  Coincident  Lodes  ;  Systems  of  Lodes  ;  Branching  and 
Intersection  of  Lodes ;  Heaving  of  Lodes ;  Contents  of 
Fissure  Veins ;  Structure  of  Fissure  Veins ;  Outcrop  of 
Lodes  ;  Gossans  ;  Association  of  Ores  in  Lodes  ;  Succession 
of  Minerals  in  Lodes  ;  Walls  of  Lodes  ;  Stockworks  .  228-254 


CHAPTER  XVII 

ORE-FORMATIONS — continued 

Bedded  Veins  or  Quasi-bedded  Ore-Formations.  Irregular  Ore- 
Formations — Masses  occupying  Cavities  ;  Metasomatic  Re- 
placement ;  Impregnations ;  Disseminations ;  Contact  Ore- 
Formations.  Origin  of  Ore-Formations — Magmatic  Segrega- 
tion Ores  ;  Magmatic  Extraction  Ores  ;  Secretionary  Ores  ; 
Sedimentary  Ores  ;  Theories  of  Lateral  Secretion  and 
Ascension  ......  255-272 


CHAPTER   XVIII 

GEOLOGICAL   SURVEYING 

Geological  Surveying.  Field  Equipment.  Topographical  Maps. 
Data  to  be  mapped.  Various  Scales  of  Maps.  Signs  and 
Symbols.  Tracing  of  Exposed  Outcrops.  Tracing  of  Con- 
cealed. Outcrjops.^Evldeace  supplied  by  Soils  and  Subsoils, 
by  Vegetation,  by  Form  of  Surface,  by  Springs,  by  Index- 
beds,  by  Alluvial  Detritus.  Carrying  Outcrops  across 
Superficial  Formations  .....  273-290 


CONTENTS  xiii 

CHAPTER  XIX 
GEOLOGICAL  SURVEYING — continued 

PAGE 

Forms  of  Outcrop.  Measurement  of  Thickness  of  Strata.  Thicken- 
ing and  Thinning  of  Strata.  Unconformity.  Overlap. 
Normal  Faults.  Reversed  Faults.  Eruptive  Rocks  and 
Contact  Metamorphism.  Regional  Metamorphism.  Archaean 
Gneissose  Rocks  ......  291-366 

CHAPTER   XX 
GEOLOGICAL  SURVEYING— continued 

Mapping  of  Unconsolidated  Tertiary  Deposits,  and  of  Glacial 
and  Fluvio-glacial  Accumulations — Boulder-clay  ;  Roches 
Moutonnees  ;  Terminal  Moraines,  etc.  Raised  Beaches. 
Lacustrine  and  Fluviatile  Deposits.  Peat  .  .  307-320 

CHAPTER  XXI 

GEOLOGICAL  MAPS   AND   SECTIONS 

Geological  Maps  and  Explanatory  Memoirs.  Geological  Sections 
— Horizontal  Sections  should  show  both  the  Form  of  the 
Ground  and  the  Geological  Structure  ;  Direction  in  which 
such  Sections  should  be  drawn  ;  Method  of  plotting  a  Section 
on  a  True  Scale.  Vertical  Sections  .  .  .  321-329 

/ 
CHAPTER  XXII 

ECONOMIC   ASPECTS   OF   GEOLOGICAL   STRUCTURE 

The  Search  for  Coal — Conditions  under  which  Coal  occurs.  Trial 
Borings.  The  Search  for  Ores — General  Considerations 
which  should  guide  the  Prospector  ;  Nature  of  the  Evidence. 
Geological  Structure  and  Engineering  Operations — Excava- 
tions, Tunnels,  Foundations  ....  330-346 

CHAPTER   XXIII 

ECONOMIC  ASPECTS   OF   GEOLOGICAL  STRUCTURE  — 
continued 

Water-supply.  Lakes  and  Impounded  Streams.  Reservoirs. 
Supply  from  Rivers.  Underground  Water — the  Water-level ; 
Natural  Springs  as  illustrating  the  course  followed  by  Sub- 
terranean Water  ;  Surface  and  Deep-seated  Springs.  Common 
Wells  and  Driven  Wells.  Artesian  Wells.  Considerations 
to  be  kept  in  view  in  the  search  for  an  Artesian  Water-supply. 
Drainage.  Distribution  of  Disease  in  relation  to  Geological 
Conditions  ......  347-367 


xiv  CONTENTS 

CHAPTER   XXIV 

SOILS  AND  SUBSOILS 

PA  OK 

Agents  of  Disintegration — Insolation  and  Deflation  ;  Rain  ;  Frost ; 
Life.  Weathering  of  Rocks.  The  Soil-cap.  Classification 
of  Soils — I.  Bed-rock  Soils,  their  Varied  Character  ;  Soils 
derived  from  Igneous,  Metamorphic,  and  Derivative  Rocks. 
II.  Drift  Soils  ;  Glacial,  Alluvial,  and  ^Eolian  Soils  .  368-392 

CHAPTER  XXV  - 

GEOLOGICAL  STRUCTURE  AND  SURFACE  FEATURES 

Denudation  and  the  Evolution  of  Surface  Features.  Mountains 
classified  according  to  Structure  and  Origin ;  Original  or 
Tectonic  Mountains  —their  Erosion  and  Transformation ; 
Subsequent  or  Relict  Mountains.  Plains  and  Plateaus  of 
Accumulation  and  Erosion.  Original  or  Tectonic  and  Sub- 
sequent or  Erosion  Valleys.  Basins.  Coast-lines  .  393-424 

APPENDICES 

APPENDIX 

A.— TABLE  OF  BRITISH  FOSSILIFEROUS  STRATA  .  .      425 

B.— THE  SCALE  OF  HARDNESS  .           .           .  .  .427 

C.— TRUE  AND  APPARENT  DIP  .           .           .  .  .427 

D.— THE  SPECIFIC  GRAVITY  OF  ROCKS          .  .  .      428 

E. — COMPASS  AND  CLINOMETER           .           .  .  429 

INDEX  .......      431 


LIST   OF    ILLUSTRATIONS 


FULL-PAGE   PLATES 

PLATE 

I.  Agate.     Rock-crystal  with  Rutile.     Garnet  . 
II.  Plagioclase  and  Orthoclase  Felspars 

III.  Microcline  and  Hornblende  . 

IV.  Augite.    Biotite.    Sphene.    Corroded  Quartz 
V.  Olivine  and  Chiastolite  .  r4*     I'-'f 

VI.  Serpentine  and  Chrysotile      .  .  . 

VII.  Structures  in  Glassy  Rocks     .  ... 

VIII.  Glassy     Rocks.      Pegmatitic    and    Ophitic 

Structures     ..... 

IX.  Granophyre.     Calcite.     Trachytic  Structure 

X.  Porphyritic  Structure,  Quartz-Porphyry 
XI.  Graphic  Granite.     Druse  in  Granite  . 
XII.  Porphyritic  Structure.     Granitoid  Structure  . 

XIII.  Spherulitic  and  banded  Obsidian 

XIV.  Orbicular  Diorite  (Corsite,  Napoleonite) 
XV.  Volcanic  Tuff  and  Veins  of  Calcite    . 

XVI.  Scoriae  or  Volcanic  Cinders    . 
XVII.  Volcanic  Bombs 
XVIII.  Section  of  Volcanic  Bomb 
XIX.  Stalagmite,  Gibraltar  .... 
XX.  Volcanic  Tuff.     Shelly  Limestone     . 
XXI.  Biotite  Gneiss.     Schistose  Conglomerate 
XXII.  Spotted  Slate.     Gneiss 

XXIII.  Gneiss  with  Phacoids. 

XXIV.  Diagonal  Bedding  in  Sandstone,  Arran 
XXV.  Rill-marks  and  Current-marks 

XXVI.  Casts  of  Sun-cracks  (Sandstone) 
XXVII.  Septarian      Nodule.        Ferruginous      Con 
cretions         ,          .„  v»    '         .   - 
XXVIII.  Dendritic  Markings    .  . 


To  face  p.  I 
^j  Between  pp.  8 
}  and  9 

To  face  p.  16 

»      17 

^  Between  pp.  24 
/         and  25 

To  face  p.  32 

»       »      33 

^  Between  pp.  40 
J          and  41 

To  face  p.  42 

»      44 

„       „      48 

„        „      49 

»        »      52 

^|  Between  pp.  56 

J          and  57 

To  face  p.  64 

„  »  65 
^  Between  pp.  72 
J  and  73 

To  face  p.  80 
»  "3 


:i 


Betweenpp.  120 
and  1  2  1 


xvi  LIST  OF  ILLUSTRATIONS 

PLATE 

XXIX.  Anticlinal  Fold,  Liddel  Water        .            .  To  face  p.  128 

XXX.  Contorted  Beds,  Ayrshire  Coast     .            .  „       „    129 

XXXI.  Contorted    Limestones,    Alps    and    Glen  ^ 

Tnt  \Between  pp.i& 

XXXII.  Contorted  Schists,  Kincardineshire            .  1  and  1V 

XXXIII.  Joints  in  Ripple-marked  Sandstone,  Fife- 

shire  Coast             ....  To  face  p.  144 

XXXIV.  Joints  in  Greywacke,  Ayrshire  Coast          .  „        „    145 
XXXV.  Tabular  Joints  in  Granite,  Goatfell,  Arran  „        „    146 

XXXVI.  Slickensides „        „    150 

XXXVII.  Columnar  Basalt,  near  Kinghorn,  Fife        .  ^  Between  pp.  152 
XXXVIII.  Curved  Joints  in  Felsite,  Arran       .            ./       0^153 
XXXIX.  Fault-rock,  Dalnacardoch,  Perthshire        .  To  face  p.  160 
XL.  Sgurr  Ruadh,  showing  Thrust-Planes         .  ^  Between  pp.  174 
XLI.  Section  of  Sgurr  Ruadh  (coloured}              .  J         and  175 
XLII.  Thrust-Plane,   Allt    Mor,    Kishorn,    Ross- 
shire           .....  To  face  p.  176 
XLI  1 1.  Junction  of  Granite  with  Gneiss.     Vein  of 

Granite      .            .            .            .            .  „        „    186 

XLIV.  Basalt  Dyke,  Kilbride  Bennan,  Arran        .  .,        ,,193 
XLV.  Dyke  Cutting   Sandstone,  Port   Leacach,  ^ 

Arran         .  .  .  .  .1  Between  pp.  200 

XLVI.  Dyke  Cutting  Volcanic  Agglomerate,  North    I         and  201 

Berwick     .  .  .  .  .  J 

XLVI  I.  Basalt  Veins,  Kingscross  Point,  Arran       .  To  face  p.  208 

XLVI  1 1.  Dykes  of  Central  Scotland.            .  „        „    209 

XLIX.  Cleavage  in  Steeply  folded  Rocks,  I  slay     .  „        „    224 

L.  Structure  of  Lodes    ....  „        „   246 

LI.  Signs  used  on  Maps  of  H.M.  Geological 

Survey        ...                         .  280 
LI  I.  Striated  Surface,  Kilchiaran,  I  slay              .  „        ,,312 
LI  1 1.  Glaciated  Surfaces,  Achnashellach,  Ross- 
shire  „        ,,314 
LIV.  Raised  Beaches.     Trees  in  Peat    .            .  „       „    318 
LV.  Skeleton  Geological  Map  with  Field  Data   \  Between  pp.  320 
LVI.  Completed  Geological  Map  with  Section   .  J  and  ^21 


LIST  OF  ILLUSTRATIONS  xvii 


ILLUSTRATIONS    IN   TEXT 

FIG.  PAGE 

1.  Crystal  of  Quartz  .......  3 

2.  Crystal  of  Rutile    .......          8 

3.  Crystal  of  Orthoclase  :  Carlsbad  Twin    .  .  .  .10 

4.  Stratification  and  Lamination      .....       108 

5.  Shales  and  Limestone       .  .  .  .  .  .no 

6.  Distribution  of  Marine  Accumulations    .  .  .  .in 

7.  Thinning-out  of  Strata      .  .  .  .  .  .112 

8.  Contemporaneous  Erosion  .  .  .  .  .113 

9.  Delta  formed  by  Torrential  Stream         .  .  .  .116 

10.  Dip  and  Strike  of  Strata  .  .  .  .  .  .128 

11.  Apparent  and  True  Dip    ......       129 

i2a.  Terminal  Curvature  in  Steeply-inclined  Strata  .  .130 

lib.  Terminal  Curvature  in  Horizontal  and  Inclined  Strata  .       130 

13.  Outcrops  concealed  under  Boulder-Clay             .            .  .131 

14.  Outcrops  concealed  under  Overlying  Strata       .            .  .131 

15.  Width  of  an  Outcrop  affected  by  Angle  of  Dip  .            .  .       131 

1 6.  Width  of  Outcrop  affected  by  Form  of  Ground  .            .  .132 

17.  Outcrop  and  Strike  ......       133 

1 8.  Strata  striking  at  each  other        .             .             .             .  -133 

19.  Monoclinal  Flexure           .            .            .            .            .  .134 

20.  Monoclinal  Fold  showing  Thinning  of  Beds  in  the  Fold  .       134 

21.  Quaquaversal  Fold            .             .             .             .             .  135 

22.  Centroclinal  Fold .            .            .            .            .            .  135 

Normal  or  Symmetrical  Folds     .             .             .            .  .136 

Model  of  Denuded  Synclinal  and  Anticlinal  Folds         .  .137 

25.  Unsymmetrical  Flexures :  Overfolds       .  .  .  .138 

26.  Isoclinal  Folds      .  .  .  .  .  .  .138 

27.  Isoclinal  Folds,  much  Denuded  .  .  .  .  .138 

28.  Recumbent  Fold    .  .  .  .  .  .  .139 

29.  Anticlinal  Double-Fold     .  .  .  .  .  139 

30.  Section  across  Mount  Blanc,  showing  Fan-shaped  Structure    .       140 

31.  Diagram  of  an  Anticlinorium       .....       140 

32.  Diagram  of  a  Synclinorium  .....       141 

33.  Normal  Faults  in  Horizontal  Strata        .  .  .  .156 

34.  Normal  Fault  in  Inclined  Strata  .  .  .  .  .156 

35.  Normal  Fault,  not  accompanied  by  Distortion  .  .  .158 

36.  Normal  Fault,  accompanied  by  Distortion          .  .  .158 

37.  Effect  produced  on  Outcrops  by  Dip-Fault         .  .  .       160 

38.  Effect  produced  on  Outcrops  by  Dip-Fault  traversing  Synclinal 

Strata      ........       160 

39.  Effect  produced  on  Outcrops  by  Dip-Fault  traversing   Anti- 

clinal Strata       .......       161 

40.  Effect  produced   on    Outcrops   by  Strike-Fault   with   Down- 

throw in  the  Direction  of  Dip  .  .  .  .  .161 


xviii  LIST  OF  ILLUSTRATIONS 

FIG-  I'AGK 

41.  Effect  produced  on    Outcrops   by   Strike-Fault,  with    Down- 

throw against  the  Dip   .  .  .  .  .  .164 

42.  Effect  produced  on  Outcrops  by  Strike- Fault  with  a  Diminish- 

ing Downthrow .......       164 

43.  Effect  produced  on  Outcrops  by  Oblique  Faults  .            •       165 

44.  Complex  Fault      .            .            .            .            .  .            .166 

45.  Step-Faults            .            .            .            .             .  .            .167 

46.  Step-Faults  hading  against  the  Dip                      .  .            .167 

47.  Trough-Faults  and  Ridge-Faults              .            .  .             .167 

48.  Shifting  of  one  Fault  by  another  .            .             .  .             .169 

49.  Intersecting  Faults           .            .            .            .  .            .169 

50.  Parallel  Faults,  with  Distorted  Strata  between  .  .            .172 

51.  Monoclinal   Flexure    passing    into    a    Normal  Strike- Fault, 

viewed  in  opposite  directions    .  .  .  .  173 

52.  Reversed  Fault  replacing  Monoclinal  Flexure   .  .  .173 

53.  Origin  of  Reversed  Faults  in  Highly  Folded  Rocks       .  .174 

54.  Escarpment  and  Outlier  .  .  .  .  .  .178 

55.  Outliers  and   Inliers    in    Conformable    and    Unconformable 

Strata      .  .  .  .  .  .  .  .179 

56.  Summit  of  an  Anticline  forming  an  I nlier  .  .  .179 

57.  I  nlier  resulting  from  Faulting       .  ..  .  .  .179 

58.  Marked  Unconformity  in  Horizontal  Strata        .  .  .180 

59.  Incidental  evidence  of  Unconformity  in  Horizonal  Strata         .       180 

60.  Strong  Unconformity        .  .  .  .  .  .182 

61.  Two  Unconformities          .  .  .  .  .  .182 

62.  Unconformity  and  Overlap  .  .  .  .  .183 

63.  Diagrammatic  Section  across  a  Batholith  .  .  .       186 

64.  Granite  Laccolith .  *  .  .  .  .  .190 

65.  Granite  Batholith  sending  out  "  Sheets "  .  .  .       190 

66.  Laccolith    ........       191 

67.  Sill  or  Intrusive  Sheet      .  .  .  .  .  193 

68.  Diagram  of  a  Sill,  showing  its  former  extension  as  a  Laccolith       195 

69.  Neck  occupied  by  Crystalline  Igneous  Rock      .  .  .       197 

70.  Neck  occupied  by  Agglomerate  .  .  .  .  .197 

71.  Neck  occupied  by  Agglomerate  and  Crystalline  Igneous  Rock       198 

72.  Cone  of   Agglomerate,   and    Neck    of    Crystalline    Igneous 

Rock       .  .  .  .  .  .  .  .      200 

73.  Prismatic  Jointing  in  a  Dyke       .        _    .  .  .  .      204 

74.  Complex  Prismatic  Jointing  in  a  Dyke   ....       204 

75.  Dyke,  showing  usual  position  of  Vapour  Pores  and  Vesicles     .       205 

76.  Composite  Dyke,  Liebenstein  (Thuringia)          .  .  .206 

77.  Veins  proceeding  from  a  mass  of  Granite  .  .       206 

78.  Effusive  Igneous  Rocks    .  .  *  .  .  .      209 

79.  Batholith  with  Aureole  of  Metamorphosed  Rocks          .  .216 

80.  Section  at  Blaafjeld          ....  .230 

81.  Sketch-Plan  of  Meinkjar,  Norway  .  .  •       230 

82.  Seams  and  Nodules  of  Clay-Ironstone  in  Carboniferous  Shales       232 


LIST  OF  ILLUSTRATIONS  xix 

FIG.  PAGE 

83.  Section  of  Auriferous  Lead  (or  Placer)  on  the  Lower  Murray, 

near  Corowa    .            .         .  ..           ,.  •          .            .             .  233 

84.  Section  across  Ore-bearing  Schists,  Urtvand  in  Dunderlands- 

tal,  N.  Norway             .            .            .            .            ...          .  235 

85.  Section   across   the    Ore-bearing   Rocks   of    Hiittenberg    in 

Carinthia           .             .             .             ...            .            .  236 

86.  Section  of  Lode   .  .  .  .   .         .  .  .'237 

87.  Simple  Lode,  showing  massive  structure       i,.            .            .  239 

88.  Complex  Lode     .             .             .             .             .             .  240 

89.  Lode  dividing  and  branching  in  Igneous  Rock  (Plan)              .  240 

90.  Transverse  Lode .  .  .  .  .  .  .241 

91.  Coincident  Lode .            .             .             .             .            .            .  241 

92.  Lode  dividing  and  re-uniting      .             .             .             ,"             .  242 

93.  Lodes  converging  and  diverging             .            .             .            .  242 

94.  Lodes  converging  and  intersecting        .             .             .  V         .  243 

95.  Lodes   intersecting  at   right   angles    without    Displacement 

(Plan)    ...                         ....  243 

96.  Contemporaneous  Cross-veins  (Plan)     ....  244 

97.  Heaving  of  one  vein  by  another             ....  244 

98.  Lamellated  Lode              .             .             .             .             ...  246 

99.  Brecciated  Lode  .......  246 

100.  Re-opening  and  refilling  of  Veins           .             .             .             .  247 

101.  Stockwork            .             .             .             .             .            .  253 

102.  Diagram-section  to  show  the  General  Structure  of  "  Saddle- 

Reefs"  ........  256 

103.  Diagram  to  show  Mode  of  Occurrence  of  Bohnerz       .            .  258 

104.  Veins  in  Limestone          ......  259 

105.  Metasomatic  Replacement  of  Limestone  by  Haematite            .  260 

106.  Reversed  Fault  in  the  Gold-bearing  Rocks  at  Johannesburg  .  262 

107.  Section.       Contact    Ore-formation    of    Goroblagodat    (Ural 

Mountains)       .......  265 

108.  Travelling  of  Soil  and  Subsoil    .....  283 

109.  Surface  features  in  Gently-folded  Sandstones   .            .            .  286 

1 10.  Form  of  Ground  influenced  by  Geological  Structure    .             .  286 
in.  Concealed  Outcrops         ......  290 

112.  Measurement  of  Inclined  Strata             ....  292 

113.  Ground-Plan  of  an  Unconformity          .            .            .            ..  294 

114.  Ground-Plan  of  Overlap              .....  295 

115.  Escarpment  and  Dip-slope          .*"...  296 

116.  Inclined  "Soft"  Rocks  overlying  "  Hard"  Rocks         .            .  297 

117.  Faulted  Strata  striking  at  each  other     ....  298 

1 1 8.  Ground-Plan  of  Neck      .  .  .  .  .  .301 

119.  Cleavage  and  Bedding    ......  302 

120.  Concealment  of  Outcrop  by  Surface  Wash        .             .            /  308 

121.  Coarse  Gravel  and  Shingle,  showing  Imbricated  Structure     .  315 

122.  Section  on  a  True  Scale— the  Horizontal  and  Vertical  Scales 

being  the  same             .            .            .            >.            .  325 


xx  LIST  OF  ILLUSTRATIONS 

HO.  PAGE 

123.  Section  across  same  area  as  in  Fig.  122 — the  Vertical  being 

three  times  greater  than  the  Horizontal  Scale           .            .  325 

124.  Diagram  Section             ......  328 

125.  Tunnel  through  Synclinal  Strata            ....  344 

126.  Heaping-up  of  Water  in  Superficial  Deposits   .            .  351 

127.  Drainage  in  Horizontal  Strata   .....  352 

128.  Drainage  in  Synclinal  Strata      .....  352 

129.  Drainage  in  Anticlinal  Strata     .....  353 

130.  Drainage  in  Massive  Igneous  Rock       ....  354 

131.  Heaping-up  of  Water  in  Igneous  Rock             .            .            .  355 

132.  Interception  of  Underground  Drainage  by  Intrusive  Rock     .  355 

133.  Interception  of  Underground  Drainage  by  Dyke         .            .  356 

134.  Interception  of  Underground  Drainage  by  Fault         .            .  356 

135.  Heaping-up  of  Water  in  Horizontal  Strata       .             .            .  357 

136.  Artesian  Wells     .......  360 

137.  Water-bearing  Beds  wedging  out  downwards  .            .            .  362 

138.  Section   across  the   Uinta   Mountains — a    Broad    Anticline 

broken  by  a  Dislocation  or  Fault       ....  397 

139.  Symmetrical  Folds  of  the  Jura  Mountains        .            .            .  397 

140.  Alpine  Types  of  Unsymmetrical  Folds  ....  398 

141.  Appalachian  Ridges  of  Pennsylvania    ....  399 

142.  Unsymmetrical  Flexures  giving  rise  to  Escarpment  Mountains  400 

143.  Section  across  the  Vosges  and  the  Black  Forest  (Penck)        .  402 

144.  Section  across  the  Wealden  Area — a  Denuded  Anticline        .  410 


PLATED 


1.  Section  of  Agate  from  an  amygdaloidal  cavity.    Nearly  natural  size. 

2.  Rock-crystal,  enclosing  needle-like  Rutile. 
3    Garnets  in  Mica-schist. 


[  To  face,  paqi 


STRUCTURAL  AND  FIELD 
GEOLOGY 

CHAPTER  I 

ROCK-FORMING   MINERALS 

Oxides — Quartz  and  its  varieties  ;  Opal ;  Specular  Iron  ;  Ilmenite  ; 
Magnetite  ;  Limonite  ;  Rutile  ;  Zircon  ;  Spinelloids  ;  Corundum  ; 
Pyrolusite,  Psilomelane,  and  Wad.  Silicates — Felspar  Group  ;  Fels- 
pathoid  Group. 

BEFORE  the  phenomena  presented  by  the  framework  of  the 
earth's  crust  can  be  fully  appreciated,  one  ought  to  have 
some  knowledge  of  rocks  and  their  various  constituents. 
This  is  all-important  for  the  student  who  is  specialising  in 
geology.  For  others  who  wish  merely  to  obtain  such  aid  in 
their  several  occupations  as  this  science  can  supply,  a  more 
moderate  acquaintance  with  minerals  and  rocks  than  the 
geologist  requires  may  suffice,  and  it  is  for  this  class  of 
students  more  especially  that  the  following  descriptions  have 
been  written.  In  these  introductory  chapters,  therefore, 
special  attention  is  paid  to  macroscopic  or  megascopic  char- 
acters— those,  namely,  which  may  be  observed  in  hand- 
specimens,  with  or  without  the  help  of  a  pocket-lens.  As  it 
is  hoped,  however,  that  some  readers  may  be  sufficiently 
interested  to  wish  to  know  more,  a  few  notes  in  smaller  type 
have  been  added,  giving  further  details  and  describing  char- 
acters which  can  only  be  studied  in  thin  slices  under  the 
microscope.  It  is  quite  a  mistake  to  suppose  that  any  great 
knowledge  of  mineralogy  is  required  to  enable  one  to  deter- 
mine the  essential  ingredients  of  a  fine-grained  rock  in  this 
way.  With  ordinary  application  one  may  in  a  short  time 

A 


v  "  STRUCTURAL  AND  FIELD  GEOLOGY 


acquire  sufficient  skill  to  diagnose  microscopically  all  the 
more  commonly-occurring  rocks  —  those,  namely,  which  are 
likely  to  come  under  the  notice  of  architects,  civil  engineers, 
agriculturists,  and  others. 

The  rock-forming  minerals  are  not  a  numerous  class,  and 
only  a  few  are  of  pre-eminent  importance.  For  example, 
the  essential  mineral  constituents  of  the  most  abundant  and 
widely  distributed  igneous  rocks  may  be  counted  on  the 
fingers.  The  components  of  common  schistose  rocks,  and  of 
the  great  class  of  derivative  rocks,  are  even  fewer  in  number. 
When  the  student  is  able  to  determine  some  twenty  minerals 
under  the  microscope,  he  should  have  little  difficulty  in 
diagnosing  most  of  the  fine-grained  rocks  that  he  is  likely  to 
meet  with.  Slight  though  this  knowledge  may  be,  it  will  yet 
enable  him  to  appreciate  what  petrographers  have  to  say  as 
to  the  genesis  of  crystalline  igneous  and  schistose  rocks,  and 
will  undoubtedly  aid  him  in  his  own  field-observations. 

For  convenience  of  description,  the  common  rock-forming 
minerals  have  been  grouped  under  the  following  heads  : 
Oxides,  Silicates,  Haloids,  Sulphides,  Carbonates,  Sulphates, 
Phosphates,  and  Elements.  As  the  minerals  included  under 
these  several  heads  are  of  very  unequal  importance,  the 
descriptions  of  the  less  significant  species  are  given  in  small 
type. 

Rock-Forming  Minerals 
I.  OXIDES 

By  far  the  most  important  rock-forming  oxide  is  silica, 
next  to  which  come  various  oxides  of  iron.  The  other  oxides 
here  described  are  of  less  frequent  occurrence  —  some  two  or 
three  being  hardly  entitled  to  rank  as  true  rock-formers. 

Quartz  is  chemically  pure  silica  (SiO2).  It  is  harder  than 
any  other  common  rock-former,  being  7  in  the  scale  of  hard- 
ness.* The  minerals  which  are  much  harder  than  quartz 
play  a  very  subordinate  part  in  rocks,  the  only  species  that 
need  be  mentioned  here  being  spinel  and  corundum.  Quartz 
has  a  specific  gravity  of  2-65,  and  when  it  assumes  a  crystalline 
form,  appears  most  frequently  as  hexagonal  prisms  terminated 
by  corresponding  pyramids  (see  Fig.  i).  Most  minerals  can  * 
*  See  Appendix  B. 


ROCK-FORMING  MINERALS 


FIG.  i. — CRYSTAL  OF 
QUARTZ. 


be  split  or  cleaved  more  readily  in  some  directions  than  in 
others.  In  certain  cases  cleavage  takes  place  in  one  direction 
only ;  in  other  cases  there  are  two  and  sometimes  three 
directions  in  which  a  mineral  may  be 
more  or  less  readily  divided.  Separa- 
tion along  such  cleavage-planes  is  some- 
times effected  with  facility,  as  in  mica, 
gypsum,  calcite,  and  fluorite — the  cleav- 
age-planes having  smooth  and  lustrous 
surfaces.  In  other  cases,  cleavage  may 
be  more  or  less  imperfect  or  even  un- 
recognisable. When  force  is  applied  to 
minerals  of  this  kind,  therefore,  they 
do  not  separate  along  planes,  but  break 
with  an  uneven  or  irregular  fracture. 
Quartz  is  one  example,  breaking,  as  it 
does,  with  a  shell-like  (conchoidal)  frac- 
ture.  In  its  purest  form  the  mineral  is 
water-clear,  and  has  a  vitreous  lustre. 
It  is  infusible  before  the  blowpipe,  and  insoluble  either  in 
hydrochloric,  sulphuric,  or  nitric  acid. 

Quartz  occurs  in  several  ways  : — i.  Frequently  it  is  a  product  of 
igneous  fusion,  being  met  with  as  an  original  constituent  of  many 
kinds  of  eruptive  rock,  such  as  granite,  quartz-porphyry,  rhyolite,  etc. 

2.  It  is  a  not  less  important  ingredient  of  many  schistose  rocks,  such 
as  gneiss,   mica-schist,   etc.,   and   is   thus    the   result  of   metamorphic 
action — the  nature  of  which  will  be  considered  in  a  subsequent  chapter. 

3.  Quartz  occurs  also  as  a   deposition   from   aqueous  solution,  and  as 
such   has   a  very  wide   distribution.     Silica   deposited  in   this   way   is 
derived  chiefly  from  the  chemical  decomposition  of  rock-forming  silicates. 
Such  solutions,  percolating  through  the  rocks  of  the  earth's  crust,  have 
brought  about  manifold   changes.     Frequently,   for   example,   we    find 
quartz  replacing  the  original  constituents  of  rocks.     Again,  many  more 
or  less    loosely-aggregated   rocks    have  been    permeated  by  siliceous 
solutions  and  converted  into  hard,  unyielding  masses.     Thus,  loose  sand 
has  been  solidified  into  sandstone,  while  sandstone,  in  its  turn,  has  been 
highly  indurated  and  changed  into  quartz-rock.     Another  result  of  the 
circulation  of  such  solutions  has  been  the  filling-up  of  cracks,  fissures, 
and  cavities  of  all   shapes  and  sizes,  in  almost  every  kind  of  rock. 
Hence,  quartz  frequently  appears  in  the  form  of  ramifying  veins  and 
veinlets,  and  is  one  of  the  commonest  minerals  associated  with   ores 
in  lodes.     4.  As  quartz  resists  decomposition  and  is  the  commonest  of 
all  rock-forming  minerals,  it   enters   conspicuously  into   the  formation 


4  STRUCTURAL  AND  FIELD  GEOLOGY 

of  a  large  number  of  sedimentary  rocks.  These,  as  we  shall  learn,  are 
simply  residual  products — that  is  to  say,  they  have  been  derived  from 
the  disintegration  and  degradation  of  pre-existing  rock-masses  ;  and 
quartz,  in  consequence  of  its  superior  durability,  its  great  abundance, 
and  wide  distribution,  naturally  forms  a  dominant  ingredient  of  con- 
glomerates, greywackes,  sandstones,  etc. 

In  coarsely  crystalline  rocks,  quartz,  even  when  it  shows 
no  external  crystalline  form,  is  quite  readily  recognised  by 
its  other  physical  characters — namely,  by  its  hardness,  its 
uneven  or  conchoidal  fracture,  its  vitreous  lustre,  and  the 
absence  of  any  trace  of  decomposition.  In  coarse-grained 
granite,  for  example,  it  appears  like  a  kind  of  transparent 
cement,  filling  up  the  straggling  spaces  between  the  other 
mineral  ingredients,  which  it  thus  seems  to  bind  together. 
In  certain  other  eruptive  rocks,  as  in  quartz-porphyry, 
pitchstone,  etc.,  it  often  occurs  as  conspicuous,  corroded, 
but  occasionally  well-formed  crystals,  disseminated  through 
a  groundmass  of  fine-grained  materials  (see  Plate  IV.  4). 
The  best-developed  quartz-crystals  met  with  in  eruptive  rocks, 
however,  are  found  in  certain  curious  irregular  cavities  which 
frequently  appear  in  granite.  The  walls  of  such  cavities  are 
usually  lined  with  fine  crystals  of  the  several  mineral  con- 
stituents of  the  rock,  amongst  which  hexagonal  prisms  and 
pyramids  of  quartz  are  commonly  prominent  (see  Plate  XL  2). 

In  finely  crystalline  rocks,  the  presence  of  quartz  can  only  be 
determined  by  microscopic  examination.  In  thin  slices  it  appears 
limpid,  water-clear,  and  quite  unaltered.  It  shows  no  trace  of  cleavage, 
but  is  traversed  by  numerous  irregular  cracks.  The  surface  appears 
smooth,  and  neither  bounding  edges  nor  internal  cracks  are  pronounced. 
When  crystals  of  the  mineral  are  present  (as  in  quartz-porphyry),  they 
usually  show  lozenge-shaped  outlines  with  rounded  angles.  According 
to  the  thickness  of  the  slice,  and  the  direction  of  the  section,  the  polari- 
sation colours  vary  in  intensity,  being  grey,  white,  yellow,  orange, 
blue,  or  green. 

Enclosures  of  other  minerals  are  common  in  quartz.  [In  large  crystals 
these  are  often  visible  to  the  naked  eye  (see  Plate  I.  2).]  Under  the 
microscope  even  the  smallest  granules  of  the  quartz  of  eruptive  rocks, 
such  as  granite,  may  appear  crowded  with  inclusions  of  rutile,  apatite, 
and  other  minerals.  The  quartz  of  granite  'also  usually  contains 
numerous  minute  fluid  cavities,  more  or  less  irregularly  disseminated 
through  the  mineral,  while  the  quartz  of  pitchstones,  rhyolites,  and 
quartz-porphyries  frequently  encloses  minute  quantities  of  glass  or  stone 
(see  Plate  IV.  4). 


ROCK-FORMING  MINERALS  5 

Varieties  of  Quartz. — The  chief  phanerocrystalline  varieties  are 
the  following : — Rock-crystal,  water-clear  ;  Avanturine,  rock-crystal, 
abundantly  spangled  with  enclosed  scales  of  mica  or  other  mineral ; 
Amethystine  Quartz,  violet-coloured  rock-crystal ;  Smoky  Quartz  includes 
dusty-brown  to  black  (Morion)  and  paler  brown  to  yellow  (Cairngorm} 
rock-crystal  ;  Milky  Quarts  is  milk-white  and  nearly  opaque,  with  a 
somewhat  greasy  lustre  ;  Common  Quartz,  not  transparent,  white,  but 
occasionally  coloured,  sometimes  occurs  with  crystalline  form,  but  is 
usually  massive.  The  most  important  cryptocrystalline  variety  of 
quartz  is  Chalcedony.  This  is  a  secondary  mineral,  which  may  occur 
in  almost  any  kind  of  siliceous  rock.  It  frequently  lines  or  fills  vesicles, 
fissures,  and  other  cavities  in  igneous  rocks,  and  is  common  in  metalli- 
ferous veins  or  lodes.  It  is  translucent,  and  has  a  somewhat  waxy 
lustre.  The  colour  varies,  the  commoner  kinds  being  white  or  grey,  but 
brown  or  black,  and  yellowish-green  and  blue  varieties  are  known.  It 
frequently  shows  a  banded  structure,  and  often  assumes  nodular, 
mammillary,  botryoidal,  reniform,  or  stalactitic  shapes,  being  obviously 
in  such  cases  a  deposition  from  aqueous  solution.  When  a  thin  slice 
of  a  spherical  concretion  is  seen  under  the  microscope,  chalcedony 
exhibits  a  finely  fibrous  radiating  texture,  and  between  crossed  nicol 
prisms  shows  a  black  cross,  which  remains  stationary  while  the  slide 
is  being  rotated.  Under  chalcedony  are  included  the  following  : — 
Carnelian,  bright  red,  but  sometimes  yellowish  ;  Chrysoprase,  apple-green  ; 
Plasma,  dark  leek-green,  but  when  spotted  with  carnelian  known  as 
Heliotrope  or  Bloodstone ;  Agate,  a  variegated  chalcedony,  the  colours 
being  either  banded  or  in  clouds,  or  due  to  visible  impurities.  In 
Banded  Agate  the  layers  are  wavy  or  zigzag,  or  concentric  and  more 
or  less  spherical,  according  to  the  conditions  of  deposition,  and  the 
shape  of  the  cavity  occupied  by  the  mineral  (Plate  I.  i).  In  Clouded 
Agate  the  variously  coloured  portions  are  irregularly  distributed.  When 
visible  impurities  in  a  chalcedony  assume  moss-like  or  dendritic  shapes, 
we  have  the  variety  known  as  Moss  Agate.  Onyx  is  an  agate  in  which 
the  coloured  layers  occur  in  even  planes  ;  when  one  of  these  is  dark 
brown,  overlaid  by  a  bluish-white  layer,  the  mineral  is  used  for  cameos — 
the  figure  being  carved  in  the  white  layer,  while  the  dark  layer  serves 
for  a  background.  Sardonyx  is  an  onyx  consisting  of  alternate  layers 
of  carnelian  and  opalescent  chalcedony.  Jasper  is  an  impure  chalcedony 
of  various  colours,  red  (due  to  ferric  oxide)  being  the  commonest.  Flint 
/is  allied  to  chalcedony,  consisting  of  cryptocrystalline  silica,  but  rendered 
opaque  owing  to  abundant  impurities  ;  it  has  a  marked  conchoidal  frac- 
ture. Chert  (including  Hornstone]  differs  little  from  flint  :  the  fracture 
is  splintery  rather  than  conchoidal.  Flint  and  chert  occur  chiefly  in 
calcareous  rocks,  in  the  form  of  nodules,  layers,  or  irregular  concretions. 
Weathering  of  Quartz  and  Chalcedony. — While  the  crystallised 
varieties  of  quartz  remain  practically  unaffected  by  the  chemical  action 
of  percolating  water,  the  cryptocrystalline  and  amorphous  forms  of  that 
mineral  are  not  so  resistant,  but  frequently  "weather"  with  a  white  crust. 
Opal  is  an  amorphous  mineral  (i.e.  devoid  both  of  external  crystalline 


6  STRUCTURAL  AND  FIELD  GEOLOGY 

form  and  internal  crystalline  structure).  It  is  composed  of  silica,  with 
a  variable  proportion  of  water  (usually  from  about  3  to  10  per  cent.); 
the  specific  gravity  of  the  mineral  (1-9  to  2-3)  is  somewhat  less  than 
that  of  quartz,  and  the  same  is  the  case  with  the  hardness  (5-5  to 
6-5).  The  texture  is  colloidal  or  jelly-like  ;  and  the  lustre  vitreous  to 
resinous.  The  colour  varies — it  may  be  white,  red,  brown,  yellow,  green, 
or  blue,  and  some  kinds  show  a  rich  play  of  colours.  Opal  usually  occurs 
in  reniform,  botryoidal,  or  stalactitic  masses,  occupying  any  irregular 
cavity  in  rocks.  In  all  cases  it  is  of  secondary  origin — that  is,  it  has 
been  subsequently  introduced  as  a  product  of  decomposition.  Many 
varieties  are  recognised,  among  which  the  following  may  be  mentioned  : 
Siliceous  Sinter  or  Geyserite,  deposited  from  thermal  waters,  often  loose 
and  earthy  ;  Hyalite,  usually  water-clear,  colourless,  but  sometimes  white 
or  translucent — it  occurs  in  the  joints,  fissures,  and  vesicular  cavities  of 
some  basalts  ;  Noble  or  Precious  Opal,  with  a  rich  play  of  colours,  met 
with  in  irregular  cavities  in  trachyte,  etc.  ;  Common  Opal,  translucent, 
but  showing  no  play  of  colours,  occurs  in  veins,  fissures,  etc.,  in  igneous 
rocks  ;  Semi-Opal  is  less  translucent  than  common  opal  ;  Jasp-Opal  is  red 
or  brown  in  colour  ;  Menilite  is  an  opaque  greyish  or  brown  concretionary 
opal,  occurring  occasionally  in  argillaceous  rocks. 

Specular  Iron  or  Haematite,  oxide  of  iron  (Fe2O3),  crystallises  in 
hexagonal  forms  (which  are  commonly  combinations  of  rhombohedra  and 
scalenohedra).  It  has  a  hardness  of  5-5  to  6-5,  and  a  specific  gravity  of 
5-19  to  5-28  ;  crystals  are  bluish  iron-grey  in  colour,  while  fibrous  forms 
are  usually  brownish-red.  The  mineral  yields  a  red  powder  when  rubbed 
with  a  steel  file.  This  red  streak  and  the  absence  of  magnetism  distin- 
guish specular  iron  from  magnetite.  It  occurs  both  crystalline  and 
massive.  The  crystalline  variety  is  common  in  veins,  and  is  often 
accompanied  by  magnetite.  Not  infrequently  it  occurs  as  an  ingredient 
of  granite,  syenite,  gneiss,  mica-schist,  phyllite,  etc.  It  is  met  with  in 
many  minerals  as  a  microscopic  inclusion  (endomorpK),  in  the  form  of 
minute  filmy  plates  or  scales,  the  presence  of  which  affects  the  colour 
of  the  including  mineral  (perimorph\  and  often  imparts  to  it  a  kind  of 
pearly  or  submetallic  glimmer  or  iridescence.  Now  and  again  it  has  been 
developed  in  limestones  at  or  near  their  point  of  contact  with  eruptive 
rocks.  It  occurs  as  a  sublimation-product  in  volcanic  regions. 

The  more  compact  or  cryptocrystalline  varieties  of  haematite  usually 
occur  as  veins,  irregular  beds,  and  masses.  Kidney-ore  is  the  name 
given  to  nodules  and  nodular  masses,  which  often  consist  of  concentric 
coats  having  a  radiating  fibrous  structure.  Haematite  frequently  occurs 
in  decomposing  igneous  rocks  as  an  alteration-product  of  ferromagnesian 
minerals,  and  it  often  coats  the  faces  of  joints  in  these  and  other  ferriferous 
rocks.  It  is  probable,  however,  that  the  ferruginous  mineral  commonly 
seen  on  joint-faces  is  in  many  cases  not  true  haematite,  but  Hydro-hematite 
or  Turgite,  which  contains  a  small  percentage  of  water— only  5  per  cent. 
In  other  respects  it  is  so  closely  similar  to  haematite  that  it  can  only  be 
differentiated  from  the  latter  by  analysis. 

Ilmenite  is  an  iron-black  mineral,  with  metallic  or  submetallic  lustre, 


ROCK-FORMING  MINERALS  7 

having  the  composition  of  FeTiO3,  and  crystallising  in  rhombohedral  forms. 
It  has  a  hardness  of  5  to  6,  a  specific  gravity  of  4-56  to  5-21,  and  the 
streak  is  black  to  reddish-brown.  It  is  practically  infusible  before  the 
blowpipe,  and  is  not  attacked  by  acids. 

This  mineral  occurs  as  massive  aggregates  in  certain  plutonic  rocks, 
especially  in  gabbros.  It  is  met  with  as  an  accessory  ingredient  in  many 
eruptive  rocks  (granite,  syenite,  gabbro,  basalt,  dolerite,  andesite),  and 
also  as  a  constituent  of  some  crystalline  schists. 

As  a  rock-constituent  ilmenite  appears  under  the  microscope  either  in 
the  form  of  rhombohedral  crystals  or  as  irregular  grains  and  patches, 
which  are  often  hard  or  impossible  to  distinguish  from  similar  aggregates 
of  magnetite,  being  like  these,  black,  opaque,  and  showing  a  metallic 
lustre.  The  mineral  is  often  altered  round  its  margins  or  even  throughout 
into  a  dull  greyish-white  opaque  substance  known  as  Leucoxene  (see 
under  Titanite}. 

Magnetite  (Fe3O4)  crystallises  usually  in  the  form  of  octahedra  or 
dodecahedra.  It  has  a  hardness  of  5-5  to  6-5,  and  a  specific  gravity  of 
4-9  to  5-2.  Its  strong  magnetism,  black  streak,  and  frequent  occurrence 
in  octahedra,  distinguish  magnetite  from  all  other  common  minerals.  It 
is  iron-grey  or  black,  like  ilmenite,  but  hardly  so  infusible,  while  it  is 
soluble  in  hydrochloric  acid.  __Ilmenite.  again,  weathers  with  a  greyish 
crust  (leucoxene)  ;  magnetite,  on  the  other  hand,  weathers  brown  (limonite). 
Magnetite  is  a  widely  distributed  rock-former,  occurring  as  large  and 
small  crystals  in  chlorite-schist  and  other  foliated  rocks.  Now  and 
again  it  is  found  associated  with  such  rocks  in  the  form  of  massive  beds 
with  a  granular  structure,  throughout  which  chromite,  ilmenite,  pyrite, 
chalcopyrite,  etc.,  are  often  abundantly  disseminated.  While  common 
in  acid  igneous  rocks,  it  is  a  still  more  frequent  (usually  microscopic) 
constituent  of  basic  igneous  rocks.  Occasionally  in  gabbros  it  occurs  as 
massive  aggregates.  It  is  met  with  also  as  a  secondary  mineral  in  many 
eruptive  rocks — a  product  of  the  alteration  of  such  ferromagnesian 
constituents  as  olivine,  augite,  hornblende,  and  biotite.  Not  being 
readily  decomposed,  magnetite  often  appears  in  alluvial  sands  derived 
from  the  disintegration  of  basic  eruptive  rocks,  etc. 

Limonite  (2Fe2O3  +  3H2O)  occurs  as  fibrous  aggregates,  assuming 
nodular,  stalactitic,  or  botryoidal  forms,  or  as  large  irregular  masses. 
Its  hardness  is  5  or  thereabout,  but  earthy  varieties  are  softer — the 
specific  gravity  =  3-4  to  3-95.  It  is  brown  or  yellowish-brown,  and  has  a 
yellow-brown  streak.  As  a  rock-constituent  it  is  always  a  product  of 
alteration — derived  from  the  decomposition  of  minerals  which  contain 
iron.  Limonite  is  itself  amorphous,  but  is  often  met  with  filling  the 
moulds  formerly  occupied  by  other  minerals,  and  thus  assuming  their 
crystalline  form  (fiseudomorphs}. 

Rutile  (TiO2)  as  a  rock-former  occurs  usually  as  minute  dark  brown  or 
reddish  grains,  pointed  prisms,  and  knee-shaped  (Fig.  2)  or  heart-shaped 
twin-crystals,  belonging  to  the  tetragonal  system.  It  is  met  with  in 
various  schistose  rocks  (gneiss,  mica-schist,  phyllite,  eclogite,  etc.). 
Needle-like  crystals  are  also  common  in  clay-slate  and  greywacke",  and 


8 


STRUCTURAL  AND  FIELD  GEOLOGY 


FIG.  2. — CRYSTAL  OF  RUTILE. 

A  knee-shaped  (geniculate)  twin. 


are  frequent  enclosures  in  such  minerals  as  rock-crystal  (Plate  I.  2)  and 
mica.  As  rutile  is  not  readily  attacked  by  the  various  agents  of  decom- 
position, it  often  survives  the  destruction  of  the  rocks  of  which  it  once 
formed  a  part,  and  is  thus  of  common  occurrence  as  grains  and  pebbles 
in  sand  and  gravel.  It  is  relatively  hard  and  heavy  ;  its  hardness  being 

6  to  6-5,  and  its  specific  gravity  4-2  to  4-3. 
It  is  infusible  before  the  blowpipe,  and 
insoluble  in  acids. 

Zircon  (ZrSiO4)  as  a  rock-former  can 
rarely  be  distinguished  by  the  naked  eye. 
Itappearsmostlyintheformofsmallbrown 
crystals  (tetragonal),  enclosed  in  other 
minerals.  Although  only  sparingly  present, 
it  has  a  wide  distribution,  occurring  in 
eruptive  rocks  of  all  kinds  (but  only  rarely 
in  the  basic  kinds),  as  well  as  in  crystalline 
schists,  especially  gneiss.  Larger  crystals 
are  found  in  some  kinds  of  syenite.  Like 
magnetite  and  rutile,  zircon  is  not  readily 
decomposed,  and  is  thus  often  met  with  in 
quartz  sands  which  have  been  derived 
from  the  disintegration  of  rocks  in  which 
the  mineral  occurs  as  a  primary  con- 
stituent. The  mineral  is  harder  and  heavier  than  rutile — the  hardness  being 
7.5  and  the  specific  gravity  4-5  to  4-7.  It  is  infusible  before  the  blow- 
pipe, and  soluble  with  difficulty  and  incompletely  in  heated  sulphuric  acid. 
Fine,  clear-coloured  varieties  (Jacynth  and  Jargoori)  are  valued  as  gems. 
Spinelloids. — These  minerals  crystallise  in  isometric  forms,  and  are 
all  (excepting  chromite)  very  hard.  They  are  not  attacked  by  acids. 
Spinel  (MgAl2O4)  has  a  hardness  of  8,  and  a  specific  gravity  of  3-5 
to  4-1.  Its  cleavage  is  imperfect.  It  is  infusible,  and  not  readily 
attacked  by  acids.  It  varies  in  colour — red,  yellow,  blue,  green,  and 
black  varieties  being  known.  It  occurs  in  some  schists  and  meta- 
morphosed limestones  and  dolomites.  [The  beautiful  coloured  transparent 
varieties  are  in  some  request  as  gems — the  red  crystals  being  known  as 
"  spinel-ruby  "  or  "  balas-ruby "  ;  the  golden-yellow  or  orange-red  as 
"  rubicelle "  ;  and  the  violet  as  "  almandine-spinel."]  Pleonaste  is  a 
greenish-black  to  black  magnesia-iron  spinel,  occurring  in  the  ejected 
limestone-blocks  of  Monte  Somma  (Vesuvius),  and  met  with  occasionally 
in  marble  and  in  the  xenoliths  or  foreign  rock-fragments  enclosed  in  basalts, 
andesites,  etc.  Picotite  is  a  dark  brown  to  black  chrome  spinel,  often 
present  in  eruptive  rocks  which  are  rich  in  olivine  (peridotites).  Chromite 
(FeCr2O4)  is  a  dark  brown  to  black  spinelloid,  of  considerable  commercial 
importance.  Its  specific  gravity  (4-5  to  4-8)  exceeds  that  of  spinel,  but 
its  hardness  (5-5)  is  considerably  less.  This  is  the  only  mineral  from 
which  salts  of  chromium  are  obtained  for  the  production  of  chrome- 
yellow  and  chrome-green.  As  a  rock-constituent,  it  is  a  common  and 
often  abundant  ingredient  of  olivine-rocks  and  serpentine. 


MICROSCOPIC  STRUCTURE  OF  MINERALS  AND  ROCKS. 


1.  Plagioclase  showing  Albite-twinning.    Gabbro.    Nicols  crossed. 

2.  Plagioclase  showing  Albite-  and  Pericline-twinning.    Diorite.    Nicols  crossed. 

3.  Twinned  crystal  of  Sanidine  (Orthoclase)  in  pseudo-spherulitic  groundmass.     Rhyolite.    Nicols  • 

crossed. 

4.  Crystals  of  Sanidine  (Orthoclase)  showing  Carlsbad-twinning  and  fluxional  arrangement.     Trachyte. 

Nicols  crossed. 


PLATE  III. 


MICROSCOPIC  STRUCTURE  OF  MINERALS  AND  ROCKS. 


1.  Microcline  with  spindle-shaped  twin  lamellse.     Granite.    Nicols  crossed. 

2.  Microcline    with    trellis-structure ;    shows    inclusions    of   Quartz  and   weathered   Orthoclase. 

Granite.     Nicols  crossed. 

3.  Euhedral  or  Idiomorphic  crystal   of  Hornblende   in   transverse   section,  with   characteristic 

outlines  and  cleavage.    Above  are  crystals  of  Augite.    Andesite. 

4.  Anhedral  or  Allotriomorphic  Hornblende,  with   characteristic  cross-cleavage.     Basal   sections 

of  Biotite  (below)  without  cleavage.    Dusty  Felspar  (above),  enclosing  two  small  prisms 
of  Apatite.     Granite. 


[Between  pages  8  and  9 


ROCK-FORMING  MINERALS  9 

Corundum  (A12O3)  crystallises  in  ^hexagonal  prisms  and  pyramids, 
but  is  often  massive.  It  is  a  very  hard  mineral  (9  in  the  scale),  and  has 
a  high  specific  gravity  (3-9  to  4).  It  occurs  sparingly  in  some  granites, 
syenites,  schists,  metamorphosed  limestones,  and  basalts.  The  beautiful 
clear-coloured  varieties  (Ruby  and  Sapphire)  are  highly  valued  as  gems. 
As  a  rock-ingredient  corundum  is  of  little  importance  ;  sometimes,  how- 
ever, it  occurs  in  considerable  masses.  Emery  (an  intimate  mixture  of 
corundum  and  magnetite  and  haematite)  occurs  in  veins  and  layers  in 
crystalline  schists. 

Pyrolusite,  Psilomelane,  and  Wad  (oxides  of  manganese)  are  also  un- 
important rock-formers,  but  they  often  appear  (particularly  psilomelane) 
as  thin  films  coating  the  walls  of  cracks  and  fissures  or  the  surfaces  of 
bedding-planes  in  various  kinds  of  rock.  The  films  often  assume  plant- 
like  forms  ("dendritic  markings,"  see  Plate  XXVIII.).  The  earthy 
varieties  of  these  oxides  occasionally  form  bedded  masses. 


II.  SILICATES 
FELSPAR  GROUP 

Felspar  is  a  general  term  for  a  number  of  closely  related 
minerals  which  play  a  very  important  role  as  rock-formers. 
They  are  the  chief  constituents  of  most  eruptive  rocks,  and  are 
met  with  likewise  more  or  less  abundantly  in  many  crystal- 
line schists.  They  vary  in  colour,  but  are  usually  grey,  white, 
or  reddish  ;  occasionally,  however,  they  show  yellow,  green, 
or  blue  tints.  As  rock-constituents  they  frequently  assume 
the  form  of  tabular  crystals,  or  appear  as  long  rods  or 
rectangular  lath-shaped  bodies.  All  are  characterised  by  two 
well-marked  sets  of  cleavage-planes  (at,  or  nearly  at,  right 
angles)  which  show  usually  a  glassy  or  pearly  lustre ; 
further,  all  have  approximately  the  same  hardness  (6  to  7),  and 
specific  gravity  (2-54  to  2-76).  Chemically,  they  are  silicates 
of  aluminium  with  either  potassium,  sodium,  or  calcium, 
or  several  of  these  together.  Hence  we  have  potash 
felspar,  soda  felspar,  lime  felspar,  soda-lime  felspar,  etc. 
These  felspars  so  closely  resemble  each  other  that  it  is  often 
hard  or  even  impossible  to  distinguish  one  from  another 
by  the  unassisted  eye.  This,  of  course,  is  especially  the 
case  when  the  crystals  are  small.  Usually,  however,  the 
particular  class  or  series  to  which  a  felspar  belongs  can 
be  determined  by  examination  in  thin  slices  under  the 


10 


STRUCTURAL  AND  FIELD  GEOLOGY 


microscope.  Two  series  of  felspars  are  recognised — one 
of  these  crystallising  in  monoclinic  and  the  other  in 
triclinic  forms.  The  monoclinic  series  includes  Orthoclase 
and  Sanidine,  while  the  triclinic  class  is  represented  by 
Microcline,  Anorthoclase,  and  Plagioclase — the  last-named 
forming  a  group  of  felspars  which  are  all  more  or  less  closely 
related,  and  often  hardly  to  be  distinguished  from  each  other 
without  careful  microscopical  or  chemical  examination.  As 
a  group  they  are  more  or  less  readily  differentiated  from  the 
monoclinic  felspars  by  the  inclination  of  their  cleavage-planes — 
in  the  monoclinic  felspars  these  planes  being  directed  at  right 
angles  to  each  other,  while  in  the  triclinic  group  referred  to 
they  are  not  at  right  angles.  Hence  we  have  two  series  of 
felspars — namely,  (a)  Orthoclase,  with  rectangular  cleavage, 
and  (ti)  Plagioclase,  with  oblique  cleavage. 

If  felspars  always  assumed  their  external  crystalline  form  and  were  of 
sufficient  size,  it  would  not  be  hard  to  distinguish  between  Orthoclase  and 
plagioclase.  As  rock-constituents,  however,  they  are  often  so  un- 
symmetric  in  shape,  or  occur  as  granules  so  small  in  size,  that  the  geolo- 
gist must  have  recourse  to  other  differentiating  characters  to  distinguish 
between  one  felspar  and  another.  Under  the  microscope,  the  plagioclase 
felspars  can  usually  be  recognised  by  their  "multiple  twinning."  A 
crystal  or  crystalline  granule  having  this  structure  appears  as  if  it  were 
composed  of  a  series  of  parallel  plates  or  lamellae. 
Twinning  is  best  seen  in  polarised  light — each 
plate  being  differently  coloured  from  its  neigh- 
bours, so  that  a  section  of  the  mineral,  if  cut  in 
the  proper  direction,  exhibits  a  banded  or  striped 
appearance  (Plate  II.  I,  2).  As  the  mineral  con- 
stituents of  a  crystalline  igneous  rock  usually  lie  at 
different  angles  and  in  different  directions,  it  does 
not  often  happen  that  the  structure  referred  to  is 
not  revealed  by  one  or  more  of  the  individual 
plagioclase  crystals,  which  are  exposed  in  the  field 
of  vision  under  a  microscope.  Even  when  the 
minerals  are  arranged  in  approximately  parallel 
layers,  as  in  the  case  of  schistose  rocks,  it  is  always 
possible  to  cut  sections  both  in  the  direction  of,  and 
across,  or  at  any  angle  to,  the  planes  of  foliation. 
Not  infrequently  the  twinned  structure  can  be  seen  by  the  naked  eye  or 
with  the  aid  of  a  pocket-lens,  when  the  felspars  are  fresh  and  not  too  small. 
The  structure  is  revealed  by  the  appearance  of  fine  parallel  lines,  with 
which  the  crystals  are  ruled  or  striated — the  lines  marking,  of  course,  the 
junction  of  separate  twin  lamellae.  The  twinning  of  Orthoclase  felspars  is 


FIG.  3.  —  CRYSTAL 
OF  ORTHOCLASE  : 
CARLSBAD  TWIN. 


ROCK-FORMING  MINERALS  11 

simple  (Fig.  3),  so  that  a  section  cut  in  the  right  direction  shows  in 
polarised  light  only  two  differently  tinted  bands  (see  Plate  II.  3,  4). 
When  the  mineral  is  not  twinned,  or  when  the  section  is  cut  parallel  to 
the  twinning  plane,  orthoclase  felspars  polarise  in  one  uniform  colour. 
The  following  are  the  more  important  felspars  : — 

Orthoclase  (monoclinic  potash  felspar,  with  high  per- 
centage of  silica).  This  mineral  is  usually  white,  grey,  or 
reddish.  It  is  not  attacked  by  ordinary  acids,  but  is  decom- 
posed by  hydrofluoric  acid,  and  fuses  before  the  blowpipe 
with  difficulty  on  thin  edges.  As  a  rock-former  it  occurs 
most  frequently  as  imperfect  crystals,  or  irregular  crystalline 
aggregates.  In  certain  igneous  rocks,  however  (as  in  quartz- 
porphyry),  it  appears  as  conspicuous  and  sometimes  well- 
formed  crystals  disseminated  among  the  finer  grained  con- 
stituents of  the  mass.  Fine  crystals  of  orthoclase  often  occur 
in  drusy  cavities  and  veins  in  granite,  and  now  and  again  in 
fissures  traversing  crystalline  schistose  rocks.  The  mineral  is 
an  essential  ingredient  of  many  eruptive  rocks  (granite, 
quartz-porphyry,  syenite,  rhyolite,  phonolite,  trachyte,  etc.). 
It  is  readily  distinguished  from  quartz  by  its  hardness, 
cleavage,  twinning,  and  frequent  turbidity — due  to  gradual 
alteration  of  the  mineral  into  kaolin. 

Sanidine  is  a  glassy  clear  variety  of  orthoclase,  usually 
much  cracked,  and  often  crowded  with  inclusions.  It  is  the 
type  of  orthoclase  which  characterises  the  younger  volcanic 
rocks — rhyolite,  trachyte,  phonolite,  etc.,  and  frequently 
assumes  the  form  of  tabular  crystals  (see  Plate  II.  3,  4). 

Adularia  is  another  clear,  transparent  orthoclase,  found  in  the  irregular 
drusy  cavities  of  some  gneisses,  and  in  fissures  in  schistose  rocks. 

Microcline  (triclinic  potash  felspar)  has  the  same  chemical  composi- 
tion as  orthoclase,  from  which  it  can  hardly  be  distinguished  without 
examination  under  the  microscope.  It  is  a  frequent  constituent  of 
granite,  appearing  often  in  well-developed  forms  in  the  drusy  cavities,  and 
the  coarsely  crystalline  veins  associated  with  that  rock.  It  occurs  also  in 
certain  syenites  and  other  eruptive  rocks  of  deep-seated  origin,  and  is 
occasionally  present  in  gneiss.  Although  it  thus  frequently  accompanies 
orthoclase  proper,  it  has  not  yet  been  met  with  in  rocks  that  contain  the 
glassy  variety  of  that  mineral — sanidine.  Under  the  microscope,  micro- 
cline  usually  shows  a  polysynthetic  structure,  due  to  the  presence  of 
minute  spindle-shaped  twin  lamellae,  so  arranged  that,  when  the  section 
is  cut  in  a  particular  direction,  it  has  in  polarised  light  a  peculiar  cross- 
hatched  appearance  (see  Plate  III.  i,  2). 


12  STRUCTURAL  AND  FIELD  GEOLOGY 

Anorthoclase  (triclinic  soda-potash  felspar)  shows  much  the  same 
structure  under  the  microscope  as  microcline,  but  usually  on  an  exceed- 
ingly minute  scale.  It  occurs  in  the  form  of  phenocrysts  with  rhombic 
outlines  in  a  well-known  igneous  rock  (rhombenporphyr)  met  with  in  S. 
Norway,  between  Christiania  and  Langesundfjord. 

The  potash  felspars  generally  weather  readily  into  kaolin. 
Not  infrequently,  however,  they  are  transformed  into  other 
minerals — muscovite  (potash  mica)  often  replacing  orthoclase. 

The  Plagioclase  felspars  sometimes  occur  in  crystalline 
masses.  As  rock-formers,  however,  they  usually  appear  as 
elongate  tabular  crystals,  not  infrequently  grouped  in  bundles 
or  forming  radiating  aggregates,  or  they  may  be  mere 
crystalline  granules.  They  form  a  series,  of  which  Albitc 
(sodium-aluminium  silicate)  and  AnortJiite  (calcium-aluminium 
silicate)  are  the  two  extremes.  The  intermediate  forms  are 
regarded  as  isomorphous  mixtures  of  these  two  silicates  in 
various  proportions,  as  shown  in  the  following  table,  where 
Ab  stands  for  albite,  and  An  for  ariorthite  : — 

Albite  (soda  felspar). 

Oligoclase :  mixture  of  Ab  and  An — the  former  predomi- 
nating. 

Andesine :  mixture  of  Ab  and  An  in  nearly  equal  propor- 
tions— Ab  slightly  predominating. 

Labradorite :  mixture  of  Ab  and  An — the  latter  slightly 
predominating. 

Bytownite :  mixture  of  Ab  and  An — the  latter  largely 
predominating. 

Anorthite  (lime  felspar). 

The  silica  percentage  ranges  from  43-16  in  anorthite  to  68-68  in  albite. 
Plagioclase  felspars  fuse  with  difficulty  before  the  blowpipe.  Anorthite 
is  decomposed  by  hydrochloric  acid  with  gelatinisation,  while  albite 
resists  ordinary  acids — the  intermediate  varieties  becoming  more 
readily  affected  the  nearer  they  approach  in  composition  to  ariorthite. 
All  are  subject  more  or  less  readily  to  alteration,  being  transformed 
especially  into  such  minerals  as  kaolin,  sericite  (a  variety  of  muscovite), 
and  epidote.  Saussurite  is  the  name  given  to  an  altered  plagioclase 
which  often  occurs  in  gabbro.  It  is  fine-grained  to  compact,  grey, 
ash-grey,  or  greenish-white,  shimmering  or  dull,  and  translucent  on  thin 
edges. 

Well-developed  and  more  or  less  perfect  crystals  of 
plagioclase  often  occur  in  the  drusy  cavities  of  eruptive  rocks ; 


ROCK-FORMING  MINERALS  13 

in  fissures  in  crystalline  schists ;  and  in  blocks  ejected  from 
volcanoes.  The  plagioclase  felspars  are  among  the  most 
important  rock-formers,  occurring  as  primary  constituents  of 
a  large  number  of  eruptive  rocks  both  as  macroscopic  and 
microscopic  individuals.  They  have  also  a  wide  distribution 
amongst  the  crystalline  schists.  The  felspars  (monoclinic  and 
triclinic  alike),  being  readily  weathered  and  decomposed,  are 
met  with  only  now  and  then  as  ingredients  of  derivative 
rocks;  they  are  not  uncommon,  however,  in  greywackes — 
especially  oligoclase. 

A  few  notes  on  the  individual  plagioclase  felspars  may  be  added  : — 

Albite  (soda  felspar)  :  usually  white  ;  resembles  orthoclase,  from  which 
it  may  be  distinguished  by  its  greater  specific  gravity  (orthoclase,  2-54 
to  2-58  ;  albite,  2-61  to  2-64)  and  the  character  of  its  twinning.  Although 
albite  frequently  appears  as  lamellar  intergrowths  in  orthoclase  and 
microcline  (=  microperthite),  yet  it  cannot  be  described  as  an  important 
constituent  of  igneous  rocks.  It  is  a  common  ingredient,  however,  of 
certain  crystalline  schists.  Now  and  again  it  occurs  as  a  "contact 
mineral "  in  argillaceous  and  calcareous  rocks,  near  their  junction  or  con- 
tact with  intrusive  eruptive  rocks. 

Oligoclase  is  a  common  constituent  of  many  eruptive  rocks,  especially 
of  those  in  which  quartz  or  orthoclase,  or  both  together,  occur  as  impor- 
tant ingredients,  as  in  syenite  and  granite.  It  is  present  likewise  in  some 
diorites,  and  in  many  of  the  porphyries  which  are  associated  with  plutonic 
rocks  ;  not  infrequently  also  in  trachytes  and  andesites  ;  and  often  in 
gneiss. 

Andesine  is  a  frequent  constituent  of  certain  eruptive  rocks,  such  as 
syenite,  tonalite,  andesite,  dolerite,  and  basalt. 

Labradorite  is  a  common  constituent  of  basic  eruptive  rocks  (gabbros, 
norites,  basalts,  diorites),  and  occurs  in  large  masses  in  certain  plutonic 
rocks,  where  it  frequently  shows  a  very  fine  play  of  colours,  due  to  the 
interposition  along  the  cleavage-planes  of  minute  scales  or  platy 
inclusions. 

Bytownite  occurs  not  uncommonly  in  andesites,  basalts,  and  other 
basic  eruptive  rocks. 

Anorthite  (lime  felspar)  occurs  not  infrequently  as  a  constituent  of 
many  basic  igneous  rocks,  as  in  some  diorites,  gabbros,  peridotites, 
basalts,  and,  less  frequently,  in  certain  andesites.  It  is  also  an  occasional 
constituent  of  metamorphic  rocks.  Fine  glassy  crystals  of  this  felspar 
occur  in  the  drusy  cavities  of  limestone-blocks  which  have  been  ejected 
from  Vesuvius. 

THE  FELSPATHOID  GROUP 
The  felspathoids  (Leucite,  Nep/ieline>  Socialite,  Hatty  ne^  and 


14  STRUCTURAL  AND  FIELD  GEOLOGY 

Noseari)  are  akin  to  the  felspars  in  chemical  composition,  and 
play  much  the  same  part  as  rock-formers.  They  are  not 
nearly  so  important,  however,  the  rocks  of  which  they  are 
characteristic  constituents  being  much  less  widely  distributed. 
They  are  restricted,  indeed,  to  a  few  igneous  rocks  mostly 
belonging  to  a  late  geological  period.  They  never  occur  as 
ingredients  of  the  crystalline  schists. 

Leucite  (silicate  of  potassium  and  aluminium)  generally  appears  in 
the  form  of  more  or  less  well-defined  single  crystals,  having  the  shape  of 
icositetrahedra  (24-faced  trapezohedra).  In  cross-sections  the  larger 
crystals  often  yield  six-sided  or  eight-sided  contours,  while  the  smaller 
crystals  are  rounded.  The  mineral  has  a  hardness  of  5-5  to  6,  and  a 
specific  gravity  of  2-45  to  2-50.  If  pure,  it  is  transparent  and  colourless, 
but  most  frequently,  owing  to  the  presence  of  impurities,  it  appears  ash- 
grey  or  greyish-yellow,  and  then  it  is  only  translucent  on  thin  edges. 
It  is  almost  infusible  before  the  blowpipe  ;  when  reduced  to  a  powder  it 
readily  dissolves  in  hydrochloric  acid,  with  separation  of  pulverulent 
silica.  Under  the  microscope,  leucite  usually  shows  abundant 
symmetrically  arranged  inclusions  of  glass,  gas  pores,  and  minute 
microlites,  grains,  etc.,  of  such  minerals  as  felspar,  augite,  and  magnetite. 
Between  crossed  nicol-prisms  it  exhibits  weak,  anomalous  double 
refraction — yielding  dark  grey  colours — the  crystals  being  traversed  by 
intersecting  alternately  light  and  dark  twin  lamellae.  This  structure, 
however,  is  not  seen  in  the  smaller  crystals,  which  are  usually  isotropic. 
The  mineral  is  readily  altered  in  nature,  becoming  white  and  opaque  as 
it  is  changed  into  zeolites  or  kaolin.  Probably  its  proneness  to  altera- 
tion is  the  reason  why  it  seldom  occurs  in  very  old  igneous  rocks.  It 
is  a  macroscopic  and  microscopic  constituent  of  certain  basic  Vesuvian 
lavas.  Similar  rocks  occur  elsewhere  in  Italy,  and  in  a  few  other 
countries. 

Nepheline  is  essentially  a  sodium-aluminium  silicate,  but  contains 
some  potassium.  Hardness,  5-5  to  6  ;  specific  gravity,  2-58  to  2-64. 
As  a  rock-constituent  it  appears  in  the  form  of  somewhat  stout  hexagonal 
prisms  with  a  glassy  lustre,  and  is  either  water-clear  or  white.  Its 
hardness  is  similar  to  that  of  leucite.  It  is  fusible  before  the  blowpipe 
with  some  difficulty  ;  and  gelatinises  with  acids.  It  is  an  essential 
constituent  of  phonolite  and  nepheline-basalt,  and  very  commonly  occurs 
in  rocks  which  contain  leucite  as  an  essential  ingredient.  Like  leucite, 
the  mineral  is  unstable  and  thus  frequently  altered  into  fibrous 
zeolites  or  muscovite.  A  dull  grey  variety  of  nepheline,  with  a  greasy 
lustre,  and  known  as  Elcsolite^  is  a  conspicuous  component  of  certain 
syenites. 

Sodalite  is  another  sodium-aluminium  silicate,  but  it  contains 
chlorine.  It  crystallises  in  isometric  forms  (dodecahedra),  and  has  a 
hardness  of  5-5,  and  a  specific  gravity  of  2-2  to  2-4.  It  fuses  to  a 


ROCK-FORMING  MINERALS  15 

colourless  glass ;  and  gelatinises  with  acids.  It  is  a  microscopic  con- 
stituent of  certain  trachytes  and  phonolites,  occasionally  appearing  as 
well-defined  crystals  in  the  vesicular  cavities  and  fissures  of  such  rocks. 
Its  common  alteration-products  are  fibrous  zeolites.  A  compact  blue 
variety,  with  a  greasy  lustre  is  a  notable  ingredient  of  elasolite-syenite. 

Haiiyne  and  Nosean  are  isomorphous  mixtures  of  sodium-aluminium 
silicate  and  calcium-aluminium  silicate,  both  with  sulphur.  Hardness, 
5-5  ;  specific  gravity,  2-28  to  2-5.  Both  minerals  are  fusible  before  the 
blowpipe ;  and  both  are  decomposed  by  acids.  They  crystallise  in 
isometric  forms.  The  larger  individuals  often  show  crystalline  con- 
tours, giving  in  microscopic  sections  more  or  less  imperfect  hexagonal 
or  quadrangular  figures.  Not  infrequently  the  crystals  present  the 
appearance  of  having  been  corroded.  The  minerals  are  hard  to 
distinguish  from  each  other,  but  haiiyne  is  usually  blue  or  bluish-green, 
while  nosean  is  generally  grey,  although  it  may  be  greenish,  brown, 
red,  or  yellow.  Microscopic  examination  shows  that  they  are  usually 
crowded  with  inclusions.  Both  minerals  are  fusible  before  the  blowpipe 
and  gelatinise  with  hydrochloric  acid.  They  occur  as  macroscopic 
and  microscopic  constituents  of  certain  igneous  rocks  that  are  rich  in 
alkalies,  such  as  phonolite.  They  are  thus  frequent  associates  of  leucite 
and  nepheline.  Both  are  apt  to  be  altered  into  fibrous  zeolites. 


CHAPTER  11 

ROCK-FORMING   MINERALS — continued 

Silicates  —  Amphibole  and  Pyroxene  Group ;  Mica  Group  ;  Olivine 
Group  ;  Chlorite  Group  ;  Talc  Group  ;  Epidote  Group  j  Garnet 
Group  ;  Tourmaline  Group  ;  Titanite  Group  ;  Andalusite  Group  ; 
Zeolite  Group  ;  Kaolinite  Group.  Haloids — Fluorite  and  Rock-Salt. 
Sulphides — Pyrite,  Pyrrhotite,  and  Marcasite.  Carbonates— Cal- 
cite,  Aragonite,  Dolomite,  and  Siderite.  Sulphates — Anhydrite, 
Gypsum,  and  Barytes.  Phosphates — Apatite,  etc.  Elements — 
Graphite. 

THE  AMPHIBOLE  AND  PYROXENE  GROUP 

THE  Amphiboles  described  here  are  calcium-magnesium 
silicates ;  others  not  referred  to  contain  soda.  Some  are  rich 
in  aluminium  and  iron,  while  others  contain  little  or  no  trace 
of  either.  When  crystallised  they  appear  as  prisms  :  but 
they  show  a  marked  tendency  to  assume  fibrous  and  radiated 
forms.  Their  specific  gravity  ranges  from  2-9  to  3-5,  and 
their  hardness  is  between  5  and  6.  They  are  usually  fusible, 
more  particularly  when  rich  in  iron. 

Amphiboles  crystallise  both  in  monoclinic  and  ortho- 
rhombic  forms,  but  only  the  former  are  important  rock- 
formers.  The  monoclinic  non-aluminous  amphiboles  are  usually 
lighter  in  colour  than  those  rich  in  aluminium  and  iron. 
The  most  commonly  occurring  representatives  of  the  non- 
aluminous  class  are  Tremolite  and  Actinolite. 

Tremolite  is  white,  grey,  or  light  green  in  colour,  and  occurs  usually 
in  the  form  of  long  blade-shaped  crystals,  striated  longitudinally  :  or  it 
assumes  the  appearance  of  thin  fibrous  crystals  radiating  from  a  centre. 
The  crystals  have  a  pearly  or  silky  lustre.  This  mineral  is  a  constituent 
of  some  schistose  rocks  ;  it  occurs  not  uncommonly  in  crystalline 
limestone  (marble)  and  dolomite  near  their  point  of  contact  with  plutonic 
rocks.  Now  and  again  it  is  met  with  as  an  alteration-product  in  olivine- 
rocks  and  serpentine.  Actinolite  differs  from  tremolite  in  containing  a 

16 


PLATE  IV. 


MICROSCOPIC  STRUCTURE  OF  MINERALS  AND  ROCKS. 


1.  Euhedral  or  Idiomorphic  crystal  of  Augite  with  prismatic  cleavage.    Basalt. 

2.  Crystals  of  Angite  in  transverse  section,  with  characteristic  outlines  and  cleavage.    Vesicular 

Basalt. 

3.  Rectangular  crystals  of  Biotite  with  parallel  cleavage;   dusty  decomposing  Felspar,  and  clear 

grains  of  Quartz.    The  lozenge-shaped  crystal  (above)  is  Sphene.    Granitite. 

4.  Corroded  crystal  of  Quartz  with  inclusions  of  glass.    Pitchstone. 


[To  face  page  16. 


PLATE  yv    _  ^_  e     ;     c 


MICROSCOPIC  STRUCTURE  OF  MINERALS  AND  ROCKS. 


1.  Idiomorphic  Olivine  in  longitudinal  section,  with  transverse  cracks  and  imperfect  cleavage. 

Basalt. 

2.  Crystal  of  Olivine  with  incipient  serpentinization  along  the  cracks  and  borders.    Basalt. 

3.  Crystals  of  Chiastolite  (Andalusite)  with  inclusions.    Slate. 

4.  Chiastolite  in  contorted  Slate. 


[To  face 


ROCK-FORMING  MINERALS  17 

considerable  percentage  of  iron  ;  hence  it  is  generally  light  or  dark  green 
in  colour.  It  usually  occurs  as  long  thin  columnar  crystals  and  radiate 
aggregates.  It  is  a  common  ingredierit  of  many  crystalline  schists, 
where  it  is  frequently  associated  with  talc,  chlorite,  and  epidote. 
In  eruptive  rocks  (as  in  saussurite-gabbro)  it  is  often  met  with  as  an 
alteration-product. 

Tremolite  and  actinolite  sometimes  assume  forms  so  fibrous  that  they 
can  be  readily  separated  into  thin,  soft,  cotton-like,  or  silky  threads,  and 
are  then  known  as  Amianthus  or  Asbestos.  The  fibres  are  often  matted 
together  so  as  to  form  felt-like  substances,  termed  "mountain-leather," 
"  mountain-cork,"  etc.  Most  of  the  asbestos  of  commerce,  however,  is 
not  amphibole,  but  fibrous  serpentine  (chrysotile). 

Of  the  mono  clinic  aluminous  amphiboles,  by  far  the  most 
important  is  Hornblende.  This  mineral  has  much  the  same 
composition  as  actinolite,  but  contains  a  notable  percentage 
of  alumina.  Two  varieties  are  recognised — namely,  Common 
Hornblende  and  Basaltic  Hornblende.  The  former  is  dark 
leek-green  to  black,  and  occurs  generally  as  long  prismatic 
crystals,  but  sometimes  as  blade-like,  fibrous,  radiating 
aggregates.  It  is  opaque  in  reflected  light,  but  usually  green 
in  transmitted  light.  It  is  an  essential  constituent  of  many 
plutonic  rocks  (syenite,  diorite,  hornblendic  granite),  occurring 
now  and  again  as  an  accessory  ingredient  in  gabbro.  It  is 
a  frequent  constituent  of  crystalline  schists  (amphibole-schist, 
hornblende-gneiss).  It  commonly  alters  to  chlorite  or  epidote, 
or  may  be  still  further  broken  up  by  weathering,  and  reduced 
to  the  condition  of  a  ferruginous  clay. 

Basaltic  Hornblende  is  generally  brownish-black  to  pitch- 
black,  but  when  viewed  in  thin  sections  it  usually  shows  a 
deep  brown  or  reddish-brown  colour.  The  crystals  are 
commonly  short,  stout  prisms,  and  are  frequently  well  formed 
(see  Plate  III.  3).  The  mineral  occurs  as  a  macroscopic  and 
microscopic  ingredient  of  certain  trachytes,  andesites,  and 
basalts — the  larger  crystals  often  showing  corroded  blackened 
borders — the  result  of  magmatic  resorption. 

The  only  other  monoclinic  amphibole  that  need  be  mentioned  is 
Smaragdite — a  peculiar  grass-green  fibrous  lamellar  form,  approaching 
actinolite  in  composition,  but  containing  a  considerable  percentage  of 
alumina.  It  occurs  in  eclogite,  where  it  forms  parallel  growths  with 
omphacite — a  similar  green  pyroxene  mineral. 

The  Pyroxenes  have  much  the  same  chemical  composi- 
tion, hardness,  and  specific  gravity  as  the  amphiboles,  and 

B 


18  STRUCTURAL  AND  FIELD  GEOLOGY 

crystallise  like  them  -in  monoclinic  and  orthorhombic  forms. 
They  differ  amongst  themselves  as  regards  fusibility — those 
containing  much  iron  being  usually  more  fusible  than 
the  less  ferriferous  varieties.  The  monoclinic  forms  are 
divisible,  like  the  corresponding  amphiboles,  into  non- 
aluminous  and  aluminous  types.  ^Th^non-aluminous pyroxenes 
are  mostly  light-coloured — white  or,  more  commonly,  some 
pale  shade  of  green.  They  occur  chiefly  in  crystalline  schists 
and  in  crystalline  limestones  and  marbles,  but  are  not  such 
important  rock-formers  as  the  corresponding  light-coloured 
amphiboles.  Their  alteration-products  t  are  usually  talc  or 
serpentine.  Of  the  aluminous  pyroxenes  the  most  notable  is 
Augite  (see  Plate  IV.  I,  2);  it  crystallises  in  prismatic  forms, 
which  are  often  twinned.  As  rock-constituents  the  crystals 
frequently  have  their  edges  and  angles  rounded  off.  Augite  is 
dark  brown  to  black,  but  in  thin  sections  may  be  almost  colour- 
less or  show  various  shades  of  brown  or  yellow,  and  sometimes 
of  green.  It  is  often  altered  into  an  aggregate  of  chlorite, 
scattered  through  which  may  be  minute  granules  of  epidote, 
calcite,  and  quartz ;  or  it  may  be  still  further  changed  to  a 
mixture  of  limonite,  quartz,  and  carbonates.  Sometimes  it 
is  replaced  by  biotite,  epidote,  calcite,  etc.  ,  It  is  an  essential 
constituent  of  such  basic  rocks  as  basalt,  dolerite,  etc.,  but 
occurs  as  an  accessory  ingredient  of  many  other  eruptive 
rocks.  Diallage  is  a  brownish,  grey,  or  greenish  variety  of 
augite,  which  rarely  assumes  a  crystalline  form,  and  has  a 
lamellar  or  foliated  structure.  Numerous  platy  inclusions 
occur  along  the  cleavage-planes,  so  that  the  mineral  exhibits  a 
submetallic  lustre  on  broken  surfaces.  It  is  an  essential  con- 
stituent of  gabbro,  and  occurs  also  as  an  occasional  ingredient 
of  serpentine  and  olivine-rocks  ;  but  appears  never  to  be  met 
with  in  effusive  igneous  rocks  (lavas). 

Omphacite  is  a  bright  green  pyroxene  occurring  in  granular  and  lamellar 
aggregates,  and  associated  with  smaragdite  and  red  garnet  in  the  rock 
known  as  eclogite. 

The  orthorhombic  pyroxenes  (hardness,^  to  6  ;  specific  gravity,  3  to 
3-5)  play  a  more  important  r61e  as  rock-formers  than  the  orthorhombic 
amphiboles.  There  are  three  types  recognised — namely,  Enstatite^ 
Bronzite,  and  Hypersthene — but  they  seem  to  be  varieties  of  one  and  the 
same  species.  Enstatite  is  a  silicate  of  magnesium  with  a  small  per- 
centage of  iron.  It  is  greenish-white  usually,  but  sometimes  darker 


ROCK-FORMING  MINERALS  19 

coloured.  It  occurs  most  frequently  in  gabbros  and  rocks  rich  in 
olivine,  generally  in  the  form  of  aggregates,  grains,  and  irregular  masses. 
Better  formed  crystals  have  been  met  with  in  certain  andesites  and 
quartz-porphyries.  Bronzite  and  Hypersthene  have  a  chemical  com- 
position similar  to  that  of  enstatite,  but  contain  a  larger  percentage  of 
iron.  Owing  to  the  occurrence  of  abundant  platy  inclusions,  bronzite 
yields  a  semi-metallic  lustre  on  broken  surfaces.  Hypersthene  is  still 
richer  in  inclusions,  and  shows  reddish  copper-coloured  reflections  due  to 
the  interposition  of  numerous  minute  dark  brown  lamellae.  Bronzite 
is  usually  dark  brown  to  reddish-brown,  but  sometimes  yellowish  or 
greenish  ;  while  hypersthene  is  much  darker — the  shades  ranging  from 
very  dark  green  or  dark  brown  to  greenish-black  and  pitch-black.  The 
two  minerals  have  much  the  same  habitat  as  enstatite,  occurring  frequently 
in  gabbros,  peridotites,  and  serpentines,  generally  without  crystal  outlines  ; 
while  well-formed  crystals  are  met  with  in  some  andesites  and  trachytes. 
It  is  by  their  behaviour  in  polarised  light  that  geologists  are  able  to 
distinguish  between  these  orthorhombic  pyroxenes.  They  are  all 
pleochroic,  and  show  a  distinct  change  of  colour  when  rotated  on  the 
microscope  stage  above  the  polariser.  The  pleochroism  seems  to 
increase  with  the  increase  of  iron — the  change  of  colour  being  feebler  in 
enstatite  and  bronzite  than  in  hypersthene.  The  rhombic  pyroxenes 
containing  little  ^ron  tend  to  be  altered  into  yellowish-green  fibrous 
serpentinous  products  termed  Bastite. 

The  two  most  important  members  of  the  amphibole  and 
pyroxene  group  are  undoubtedly  hornblende  and  augite,  and 
as  these  minerals  are  often  hard  to  distinguish,  it  may  be 
useful  to  add  a  few  notes  on  the  characters  by  which  they 
can  be  recognised  : — 

First,  as  regards  habitat,  the  rule  is  that  Common 
Hornblende  most  frequently  occurs  in  rocks  containing  a 
considerable  percentage  of  silica,  and  is  thus  often  associated 
with  quartz  and  highly  silicated  felspars,  as  orthoclase,  albite, 
oligoclase.  Basaltic  Hornblende,  on  the  other  hand,  occurs 
as  an  accessory  ingredient  chiefly  in  basic  and  intermediate 
eruptive  rocks,  as  in  many  basalts,  andesites,  and  trachytes. 
Augite  is  an  essential  constituent  of  basalts,  and  dolerites, 
and  a  common  ingredient  of  some  trachytes,  andesites,  etc. ; 
only  very  rarely  a  pale  augite  has  been  met  with  in  granite. 
So  that  we  may  say  the  home  of  common  hornblende  is 
chiefly  in  acid  plutonic  rocks  and  crystalline  schists ;  while 
basaltic  hornblende  and  augite  are  confined  mostly  to  eruptive 
rocks  not  rich  in  silica. 


20  STRUCTURAL  AND  FIELD  GEOLOGY 

In  thin  slices  under  the  microscope  it  is  generally  easy  to  distinguish 
between  hornblende  and  augite.  The  faces  of  the  unit  prism  in  horn- 
blende are  inclined  to  each  other  at  an  angle  of  124°  30',  while  in  augite 
the  corresponding  angles  are  87°  6',  or  very  nearly  a  right  angle.  The 
cleavage-planes  being  in  the  direction  of  these  faces,  it  is  obvious  that 
those  of  augite  must  intersect  at  nearly  90°,  while  the  angles  between  the 
two  directions  of  cleavage  in  hornblende,  in  transverse  sections  of  the 
crystal,  will  be  124°  30'  and  55°  30'  (see  Plates  III.,  IV.).  The  cleavage 
in  hornblende  is  usually  more  marked  than  in  augite.  Again,  hornblende 
is  very  distinctly  pleochroic,  while  in  augite  the  change  of  colour  is 
usually  feeble,  and  often  altogether  wanting. 

THE  MICA  GROUP 

The  micas,  as  rock-formers,  mostly  occur  as  thin  plates 
and  scales,  the  surfaces  of  which  show  a  pearly  to  submetallic 
lustre.  Usually  these  plates  are  irregular  in  shape,  but  now 
and  again  they  are  six-sided.  The  micas,  however,  are  really 
monoclinic  with  pseudo-hexagonal  symmetry.  The  cleavage 
is  perfect,  all  micas  being  readily  split  up  into  exceedingly 
thin,  transparent,  and  elastic  leaflets.  They  are  all  rather 
soft  (2-5  to  4  in  the  scale),  and  the  specific  gravity  ranges 
from  2-7  to  3.  They  are  essentially  silicates  of  aluminium 
and  potassium  (or  sodium),  some  kinds  containing  magnesium 
and  iron.  Only  two  micas  are  important  rock-formers,  namely, 
the  brown  to  black  Biotite  or  ferromagnesian  mica,  and  the 
silver-white  Muscovite  or  potash  mica.  They  are  essential 
constituents  of  many  schistose  rocks  and  of  granite,  and  are 
met  with  in  a  large  number  of  eruptive  rocks  of  all  ages. 
Soft,  non-elastic  scales  of  mica  are  also  of  common  occurrence 
in  many  derivative  rocks,  particularly  in  fissile  sandstones. 

Biotite  (ferromagnesian  mica)  is  usually  dark  brown  to  black,  but 
green  and  red  varieties  are  known.  It  is  decomposed  by  strong  sul- 
phuric acid ;  and  in  nature  alters  readily  to  chlorite,  with  separation  of 
iron-oxide.  Not  infrequently,  however,  biotite  becomes  pale  through  loss 
of  iron,  and  then  assumes  a  golden  yellow  to  silver-grey  colour,  thus 
sometimes  closely  resembling  muscovite.  It  is  a  primary  or  original 
constituent  of  granites,  rhyolites,  some  syenites  and  diorites,  trachytes, 
etc.  In  effusive  rocks  the  scales  often  show  blackened  borders,  which,  as 
in  the  case  of  basaltic  hornblende,  appear  to  be  due  to  the  corrosive 
action  of  the  igneous  magma.  Biotite  occurs  also  in  certain  schistose 
rocks.  Being  a  less  durable  mineral  than  muscovite,  it  is  not  so  often  met 
with  in  sedimentary  rocks.  In  thin  rock-sections  under  the  microscope, 


ROCK-FORMING  MINERALS  21 

biotite,  if  cut  at  right  angles  to  its  vertical  axis  (or,  in  other  words,  if  the 
slice  be  parallel  to  the  cleavage-planes),  appears  deep  brown  or  deep  green 
to  black,  and  shows  little  or  no  change  of  colour  when  rotated  above  the 
polariser.  But  when  the  section  cuts  across  the  cleavage-planes,  which 
then  appear  as  a  series  of  parallel  lines  traversing  the  mica,  as  shown  in 
Plate  IV.  3,  and  the  stage  of  the  microscope  is  rotated,  the  change  of 
colour  is  strongly  pronounced.  The  polarisation  colours  are  very 
brilliant  in  sections  showing  cleavage,  and  cut  thin  enough.  Inclusions 
are  frequently  numerous,  mostly  of  apatite  and  magnetite,  and  less 
commonly  of  zircon  and  rutile. 

Muscovite  (potash  mica)  is  sometimes  colourless,  but  usually  pale- 
coloured  or  silvery ;  occasionally,  however,  it  assumes  a  pale  shade  of 
brown  or  green.  It  fuses  on  thin  edges  to  a  grey  glass  or  white  enamel, 
but  is  not  attacked  by  acids,  and  as  a  rock-constituent  is  not  so  readily 
altered  as  biotite.  As  a  primary  rock-former  its  chief  habitats  are  the 
crystalline  schistose  rocks  (gneiss,  mica-schist,  phyllite),  and  the  granites. 
It  never  occurs  as  an  original  constituent  in  any  igneous  rocks  save 
granite,  certain  quartz-porphyries,  and  syenites.  Being  a  mineral  not 
readily  decomposed,  it  frequently  appears  in  the  form  of  soft,  worn- 
looking,  non-elastic  scales  in  sedimentary  rocks  of  many  kinds.  Although 
muscovite  has  no  great  range  as  a  primary  constituent  of  crystalline 
eruptive  rocks,  it  occurs  in  many  as  a  secondary  ingredient — the  product 
of  the  alteration  of  silicates  rich  in  alumina.  Thus  it  often  replaces  such 
minerals  as  andalusite,  felspar,  nepheline,  etc.  Seen  in  thin  sections 
under  the  microscope,  muscovite  is  colourless  or  very  faintly  yellowish 
or  light  green.  It  shows  no  change  of  colour,  or  at  most  only  a  slight 
difference  in  the  intensity  of  the  colour,  when  rotated  above  the  polariser. 
It  polarises,  however,  more  brilliantly  than  biotite.  Inclusions  are  few. 

Although  the  micas,  as  rock-formers,  occur  most  frequently  in  the 
form  of  scales,  flakes,  or  plates  of  relatively  small  size,  they  now  and 
again  appear  as  large  rough  prisms,  often  tapering  to  a  point — as,  for 
example,  in  limestones  which  have  been  subject  to  metamorphic  action. 
Very  large  individuals  of  muscovite  also  are  met  with  in  the  pegmatitic 
veins  (giant  granite)  associated  with  so  many  granitic  masses. 

THE  OLIVINE  GROUP 

The  minerals  of  this  group  are  non-aluminous  silicates. 
The  only  one  of  importance  as  a  rock-former  is  Olivine 
(Peridote) — a  silicate  of  magnesium  and  iron  which  crystal- 
lises in  orthorhombic  forms  and  shows  an  imperfect  cleavage. 
It  has  a  hardness  of  6-5  to  7,  and  a  specific  gravity  of  3  to  4. 
The  proportion  of  iron  varies — specimens  containing  very 
little  being  infusible,  while  those  which  are  rich  in  iron  are 
more  or  less  readily  fused.  The  mineral  is  slowly  decomposed 
by  cold  hydrochloric  acid  with  gelatinisation.  It  is  usually 


22  STRUCTURAL  AND  FIELD  GEOLOGY 

yellowish-green  or  olive-green,  has  a  glassy  lustre,  and  breaks 
with  a  conchoidal  fracture.  As  a  rock-former  it  sometimes 
constitutes  the  whole  mass  or  the  larger  proportion  of  a  rock, 
as  in  dunites  (peridotites).  It  is  present  also  in  many  other 
igneous  rocks — more  especially  in  those  of  basic  and  inter- 
mediate composition,  as  certain  gabbros,  basalts,  and  fels- 
pathoid  rocks.  It  is  readily  recognised  in  such  rocks  by  the 
naked  eye  as  granules  or  blebs,  usually  of  a  greenish  tint 
with  a  glassy  lustre,  and  showing  its  conchoidal  fracture. 
Now  and  again  it  occurs  in  basalts  as  large  granular  aggre- 
gates resembling  nodules,  some  of  which  may  measure  5  or 
6  inches  across,  but  they  are  generally  smaller.  Forsterite, 
a  light-coloured  variety,  is  met  with  as  a  "  contact  mineral "  in 
metamorphosed  limestones.  In  nature,  olivine  alters  readily  to 
serpentine ;  probably,  indeed,  most  serpentines  have  originated 
from  the  alteration  of  olivine-rocks.  The  finely  coloured 
(yellow  or  green)  transparent  varieties  of  olivine  are  used  in 
jewellery,  and  are  known  as  Chrysolite  and  Peridote. 

In  thin  rock-slices  olivine  is  usually  almost  colourless,  but  may  show 
pale  yellowish-green  or  yellowish-brown  tints.  In  basic  eruptive  rocks 
it  appears  sometimes  in  good  crystal  forms,  with  lozenge -shaped  or  long 
rectangular  outlines  (see  Plate  V.  I,  2),  but  the  outlines  are  frequently 
rounded  as  if  from  magmatic  corrosion.  It  shows  high  relief,  the  out- 
lines of  the  mineral  and  the  cracks  traversing  it  being  strongly 
pronounced.  It  is  not  pleochroic,  but  polarises  rather  brilliantly. 

THE  CHLORITE  GROUP 

Under  this  head  are  included  certain  greenish  coloured 
minerals  which  are  composed  essentially  of  hydrated  silicate 
of  magnesium  and  aluminium,  usually  with  some  iron.  As  a 
rock-former  Chlorite  occurs  in  the  form  of  pseudo-hexagonal 
non-elastic  plates,  but  most  frequently  as  bent  and  irregularly 
bounded  scales,  tufts,  and  fibres,  or  as  scaly  or  earthy 
aggregates.  Often  it  somewhat  resembles  mica.  The  hard- 
ness is  2  to  3,  and  the  specific  gravity  2-6  to  2-8.  The  only 
rock  largely  composed  of  this  mineral  is  chlorite-schist  It 
occurs  frequently,  however,  in  eruptive  rocks  as  a  secondary 
product,  from  the  alteration  of  such  minerals  as  hornblende, 
augite,  biotite,  etc.  Many  igneous  rocks,  indeed,  owe  their 


ROCK-FORMING  MINERALS  23 

greenish  colour  to  the  alteration  of  their  original  ferro- 
magnesian  constituents  into  chlorite.  [Here  also  may  be 
included  Glauconite — a  hydrous  silicate  of  aluminium,  potas- 
sium, and  iron — which  occurs  in  the  form  of  small  rounded 
granules  of  a  greenish  colour  in  certain  sandstones  of 
Cretaceous  and  Tertiary  age ;  it  is  also  met  with  in  amyg- 
daloidal  cavities  in  igneous  rocks.] 

THE  TALC  AND  SERPENTINE  GROUP 

Talc  (hydrous  silicate  of  magnesium)  is  a  white  or  pale 
greenish  mineral,  readily  cleavable  into  non-elastic  folia,  and 
so  soft  that  it  can  be  scratched  with  the  finger-nail.  Hard- 
ness =  i ,  specific  gravity  =  2-7  to  2-8.  It  has  a  pearly  lustre  and 
a  pronounced  greasy  feel ;  is  fusible  on  thin  edges  to  a  white 
enamel,  but  is  not  decomposed  by  acids.  It  never  assumes 
a  crystalline  form.  In  igneous  rocks  it  occurs  rarely,  and 
always  as  a  secondary  product,  usually  in  the  form  of  foliated 
plates  and  scales,  replacing  non-aluminous  magnesian  silicates. 
It  is  met  with  chiefly  in  the  crystalline  schists,  being  the  chief 
ingredient  of  talc-schist 

Steatite  (soap-stone)  is  a  cryptocrystalline  to  compact  variety  of  talc. 
Potstone  is  another  but  very  impure  variety.  Sepiolite  or  Meerschaum  is 
a  closely  allied  mineral  of  essentially  the  same  chemical  composition.  It 
is  amorphous,  occurring  in  irregular  shaped  nodules  and  masses,  which 
are  compact  and  finely  porous.  When  dry  it  floats  in  water,  which  it 
absorbs  greedily.  Like  talc,  it  is  eminently  a  product  of  the  alteration  of 
magnesian  silicates.  As  a  rock-former  it  is  of  no  importance. 

Serpentine  (hydrous  silicate  of  magnesium,  often  contain- 
ing iron;  hardness  =  3  to  4;  specific  gravity  =  2-5  to  2-7), 
like  talc,  never  assumes  a  crystalline  form,  but  occurs  in 
compact  or  granular  masses  and  in  aggregates  with  a  lamellar, 
scaly,  or  fibrous  structure.  The  colour  is  some  dark  shade  of 
green,  red,  or  yellow,  often  mottled  or  variegated.  The  finely 
fibrous  variety  is  known  as  Chrysotile  (see  Plate  VI.).  [Most 
of  the  "  asbestos "  of  commerce  is  not  true  asbestos,  but 
chrysotile.]  Serpentine  is  fusible  with  difficulty  on  thin 
edges,  and  is  decomposed  by  hydrochloric  acid.  It  is  always 
a  secondary  mineral — a  product  of  the  alteration  of  ferro- 
magnesian  minerals,  as  olivine,  pyroxenes,  amphiboles,  etc. 


24  STRUCTURAL  AND  FIELD  GEOLOGY 

It  is  the  chief  constituent  of  the  rock  serpentine.  Noble 
Serpentine  is  a  pure  variety  of  a  uniform  colour  (green  or 
yellow),  which  takes  on  a  fine  polish,  and  is  used  as  an 
ornamental  stone. 

THE  EPIDOTE  GROUP 

The  principal  rock-forming  member  of  this  group  is  Pistazite,  or 
iron-epidote — so  called  to  distinguish  it  from  Zoisite^  or  lime-epidote. 
Pistazite  is  a  silicate  of  calcium,  aluminium,  and  iron,  which  occurs 
crystallised  in  monoclinic  forms,  or  appears  in  finely  granular  masses 
of  a  peculiar  pistachio-green  colour.  The  hardness  is  from  6  to  7,  and 
the  specific  gravity  3-2  to  3-5.  The  mineral  fuses  with  difficulty  before 
the  blowpipe,  and  is  partially  decomposed  by  hydrochloric  acid.  It  is 
met  with  frequently  as  a  constituent  of  schistose  rocks  (epidote-gneiss, 
epidote-amphibolite),  and  as  a  "contact  mineral"  in  limestones,  etc., 
which  have  been  affected  by  the  intrusion  of  igneous  rock.  It  is  a 
common  alteration-product  in  eruptive  rocks,  replacing  such  minerals  as 
hornblende,  biotite,  felspars,  etc.,  and  often  associated  with  chlorite. 
Zoisite  (orthorhombic)  is  a  silicate  of  calcium  and  aluminium,  met  with 
not  infrequently  in  schistose  rocks  and  as  an  alteration-product  of 
felspar  in  gabbro.  \Clinozoisite  is  a  monoclinic  epidote  containing  little 
iron,  and  approaching  to  zoisite  in  composition.] 

THE  GARNET  GROUP 

Garnets  are  silicates  of  aluminium,  iron,  calcium,  magnesium, 
chromium,  and  manganese,  usually  only  two  or  three  of  these  being 
abundantly  present.  According  to  the  dominance  of  the  chief  con- 
stituents, we  have  iron-,  calcium-,  magnesium-,  manganese-aluminium 
garnets,  etc.  They  usually  assume  dodecahedral  (see  Plate  I.  3)  or 
icositetrahedral  forms,  and  have  an  imperfect  cleavage.  The  hardness  is 
6-5  to  7-5,  and  the  specific  gravity  3-4  to  4-3.  The  lustre  is  greasy  or 
resinous.  Common  rock-forming  iron-aluminium  garnet  is  generally 
some  shade  of  red — hyacinth  to  reddish-brown.  It  is  fusible  before  the 
blowpipe,  but  is  not  readily  decomposed  by  acids.  In  nature  it  alters 
chiefly  to  chlorite,  and  sometimes  to  serpentine,  epidote,  etc.  Under  the 
microscope  its  sections  are  rounded  or  many-sided  ;  it  shows  no  cleavage, 
but  irregular  cracks  which  are  not  infrequently  lined  with  decomposition- 
products.  Enclosures  often  abound.  Usually  garnet  remains  dark  when 
rotated  between  crossed  nicols.  It  is  common  in  many  schists,  is  an 
essential  constituent  of  eclogite  and  garnet-rocks,  and  occasionally  occurs 
in  granite  and  quartz-porphyry.  Calcium-aluminium  garnet  is  often 
present  as  a  "contact  mineral"  in  metamorphosed  limestone.  The  clear, 
finely  coloured  varieties  of  garnet  have  some  value  as  gems  ;  amongst 
these  are  Almandine  (iron-aluminium  garnet)  and  Pyrope  (magnesium- 
aluminium  garnet),  the  former  occurring  in  schists  and  granite,  the  latter 
in  peridotites  and  serpentine.  Melanite,  a  black  calcium-iron  garnet,  is 
met  with  in  some  trachytes,  phonolites,  and  other  volcanic  rocks. 


PLAJIE  yi. 


SERPENTINE  VEINED  WITH  CHRYSOTILE.    Natural  size. 


n. 


MICROSCOPIC  STRUCTURE  OF  MINERALS  AND  ROCKS. 


1.  Crystallites  and  Microlites  in  glassy  base.    Pitchstone. 

2.  Lath-shaped  crystals  of  Felspar  in  glassy  base.    Andesite. 

3.  Spherulites  in  clear  glass.    Obsidian. 

4.  Perlitic  structure.    Pitchstone. 


[Between  pages  24  and  25. 


ROCK-FORMING  MINERALS  25 

Cordierite  (Dichroite,  lolite)  is  a  magnesium-aluminium  silicate,  con- 
taining a  little  iron.  It  crystallises  in  orthorhombic  forms.  Hardness 
=  7  to  7-5  ;  specific  gravity  =  2-59  to  2-66.  It  is  usually  some  shade  of 
blue,  has  a  vitreous  lustre,  is  hardly  attacked  by  acids,  and  fuses  with 
difficulty  before  the  blowpipe.  It  occurs  in  many  schistose  rocks, 
especially  in  gneisses,  in  eruptive  rocks  (granites,  quartz-porphyries, 
rhyolites,  andesites),  and  as  a  product  of  contact-metamorphism,  in  the 
form  of  granular  aggregates,  in  the  rocks  known  as  "  hornfels." 

THE  TOURMALINE  GROUP 

Tourmaline  crystallises  in  rhombohedral  (hemirnorphic)  forms.  It  has 
a  complicated  and  variable  chemical  composition,  but  is  essentially  a 
borosilicate  of  aluminium,  with  magnesium,  iron,  alkalies,  fluorine,  and 
basic  water.  Its  hardness  is  7  to  7-5,  and  specific  gravity  2-94  to  3-24.  It 
is  not  attacked  by  acids,  but  is  fusible,  the  degree  of  fusibility  varying  with 
the  chemical  composition.  The  only  form  of  any  importance  as  a  rock- 
constituent  is  the  black  variety,  Schorl,  which  often  occurs  as  long 
trigonal  prisms  longitudinally  striated ;  appears  also  as  microscopic 
prisms  and  grains,  or  as  groups  of  acicular  crystals  with  a  radiated 
arrangement ;  occasionally  it  is  met  with  as  massive  aggregates.  It 
varies  from  very  dark  green  to  black.  The  cleavage  is  indistinct,  and  this, 
with  its  greater  hardness  and  the  form  of  the  prisms,  serves  to  distinguish 
schorl  from  hornblende,  which  it  often  resembles  in  colour  and  pleo- 
chroism.  It  is  often  a  constituent  of  schistose  rocks,  and  not  infrequently 
an  ingredient  of  acid  plutonic  rocks,  especially  granite.  It  occurs 
commonly  as  a  "  contact  mineral "  in  the  zone  of  altered  rocks  surround- 
ing granite,  etc.  The  mineral  does  not  weather  readily.  The  trans- 
parent, beautifully  coloured  tourmalines  are  in  some  request  for 
jewellery. 

THE  TITANITE  GROUP 

Titanite  or  Sphene  is  really  the  only  member  of  this  group,  the  others 
being  merely  varieties  of  the  same  mineral.  As  a  rock-former  titanite  is 
a  widely  distributed  accessory  ingredient  of  eruptive  rocks  (especially  of 
hornblendic  granite,  syenite,  diorite,  etc.),  and  occurs  also  in  certain 
schistose  rocks  and  crystalline  limestones.  It  is  a  silicate  and  titanate  of 
calcium,  crystallising  in  monoclinic  forms,  which  are  usually  lozenge-  or 
wedge-shaped  (see  Plate  IV.  3).  It  is  decomposed  by  sulphuric  and  hydro- 
fluoric acids,  and  fuses  with  difficulty.  Its  hardness  is  5  to  5-5,  and  its 
specific  gravity  3-4  to  3-6.  Its  colour  is  yellowish  to  brown.  Well-formed 
crystals  often  appear  in  the  drusy  cavities  of  granite,  in  gneiss,  and  in 
metamorphosed  limestones.  As  an  accessory  ingredient  of  eruptive 
rocks  it  is  usually  of  microscopic  size.  Leucoxene  is  a  dull  white  or  grey 
earthy  form  of  titanite,  which  occurs  as  an  alteration-product  of  ilmenite. 
Sphene  is  a  somewhat  stable  mineral,  being  not  readily  weathered. 

THE  ANDALUSITE  GROUP 
These  are  silicates  of  aluminium,  crystallising  in  orthorhombic  and 


26  STRUCTURAL  AND  FIELD  GEOLOGY 

triclinic  forms,  and  occurring  chiefly  in  crystalline  schists  and  in  rocks 
which  have  been  affected  by  the  action  of  intrusive  masses.  The 
members  of  the  group  of  most  frequent  occurrence  as  rock-formers  are 
Andalusite  (with  its  variety  Chiastolite\  Sillimanite^  Kyanite^  and 
Staurolite.  The  first  three  are  silicates  of  aluminium  alone,  while 
staurolite  contains  iron  and  magnesium  in  addition. 

Andalusite  occurs  not  infrequently  as  well-developed  columnar  prisms 
(orthorhombic)  in  mica-schist  and  gneiss,  and  is  often  a  notable 
ingredient  of  the  altered  rocks  surrounding  a  mass  of  granite.  Hardness 
7  to  7-5  ;  specific  gravity  2-94  to  3-2.  It  is  usually  more  or  less  crowded 
with  inclusions  ;  and  when  these  are  regularly  arranged  so  as  in  cross- 
sections  of  the  prism  to  show  a  cruciform  or  tesselated  pattern,  we  have 
the  variety  known  as  Chiastolite  (see  Plate  V.  3,  4).  This  variety  is  of 
common  occurrence  in  argillaceous  rocks,  which  have  been  affected  by 
intrusions  of  granite.  Sillimanite  assumes  the  form  of  thin  rod-like  or 
needle-like  orthorhombic  prisms,  occurring  sometimes  under  the  same 
conditions  as  chiastolite,  but  met  with  chiefly  in  crystalline  schists.  The 
finely  fibrous  aggregates  appearing  in  the  form  of  lenticular  lumps  are 
known  as  Fibrolite.  Kyanite  is  a  white  or  pale  blue  mineral,  crystallising 
in  long  broad  flattened  prisms  (triclinic),  and  occurring  in  certain 
crystalline  schists,  but  never  in  igneous  rocks.  A  remarkable  character 
of  kyanite  is  its  hardness,  which  is  not  the  same  in  different  directions  ; 
along  the  broad  lateral  planes  it  is  only  5,  while  across  these  it  is  7.  It 
is  often  associated  with  garnet  and  Staurolite — the  latter  being  a  dark 
brownish-red  mineral  which  assumes  the  form  of  short  and  thick  or  long 
and  broad  columnar  crystals  (orthorhombic).  Interpenetrating  cruciform 
twins  of  staurolite  are  very  common.  It  does  not  occur  in  eruptive 
rocks.  This  mineral  is  not  readily  weathered. 

THE  ZEOLITE  GROUP 

The  Zeolites  are  essentially  decomposition-products,  and  frequently 
occur  in  igneous  rocks  as  the  result  of  the  alteration  of  certain  original 
constituents.  They  are  all  hydrated  silicates  of  alumina  and  alkalies 
with,  in  many  cases,  lime.  As  a  rule  they  are  colourless,  and  usually 
transparent  to  translucent.  The  water  they  contain  is  readily  driven  off 
before  the  blowpipe,  and  most  of  them  are  easily  decomposed  by  acids. 
They  occur  chiefly  in  the  vesicular  cavities  and  fissures  of  eruptive  rocks, 
or  they  may  replace  some  of  the  original  constituents  of  these  rocks,  more 
especially  the  felspars  and  felspathoids.  Among  the  more  commonly 
occurring  species  are  Analcite,  Stilbite,  Natrolite,  Chabazite. 

THE  KAOLINITE  GROUP 

Various  decomposition-products  may  be  included  in  this 
group,  of  which  much  the  most  important  is  the  hydrated 
silicate  of  alumina — Kaolinite.  When  pure  this  mineral  is 
usually  white,  earthy,  or  mealy.  Occasionally,  under  the 


ROCK-FORMING  MINERALS  27 

microscope,  this  white  powder  may  be  seen  to  consist  largely 
or  entirely  of  minute  transparent  or  translucent  plates,  with 
pseudo-hexagonal  symmetry.  Before  the  blowpipe  it  is 
infusible,  and  is  insoluble  in  acids ;  hardness  =  I  ;  specific 
gravity  =  2- 5.  It  is  a  common  alteration-product  of  many 
rock-forming  aluminous  silicates,  notably  orthoclase,  albite, 
and  lime-soda  felspars.  When  moistened  with  water,  it  is 
highly  plastic.  Impurities  are  usually  present,  particularly 
iron-oxides,  which  give  it  a  yellow,  red,  or  brown  colour; 
other  colours  met  with  are  grey,  blue,  and  green.  Lithomarge 
is  merely  an  impure  compact  kaolin ;  it  is  often  mottled  red 
owing  to  the  presence  of  ferric  hydrate. 

III.    HALOIDS 

Fluor-spar  or  Fluorite  (calcium  fluoride)  is  hardly  entitled  to  be 
called  a  rock-former.  It  occurs  rarely  as  rounded  grains  in  granitic, 
syenitic,  and  gneissic  rocks,  where  it  is  apparently  of  secondary  origin. 
It  is  met  with,  however,  frequently  as  a  gangue-mineral  in  lodes, 
particularly  in  association  with  lead-  and  tin-ores.  The  common  form  of 
the  crystallised  mineral  is  a  cube,  and  interpenetrating  twins  often  occur. 
The  colour  is  variable — violet,  blue,  green,  yellow,  and  occasionally  pink. 
Thick  veins  of  granular  fluor-spar  appear  now  and  again,  traversing 
crystalline  schistose  rocks,  especially  in  the  neighbourhood  of  granite 
masses.  The  mineral  has  a  hardness  of  4  and  a  specific  gravity  of  3-2. 
It  is  decomposed  by  sulphuric  acid,  but  hardly  attacked  by  other  acids, 
and  fuses  with  some  difficulty  before  the  blowpipe. 

Rock-salt  (sodium  chloride)  crystallises  in  the  form  of  cubes,  but  occurs 
massive  as  a  rock  in  beds,  associated  with  anhydrite  and  gypsum.  It 
is  met  with  also  as  a  product  of  sublimation  in  volcanic  regions,  along  with 
calcium-  and  magnesium-chlorides  and  calcium-sulphate. 

IV.    SULPHIDES 

Pyrite  (disulphide  of  iron)  commonly  crystallises  in  cubes 
and  octahedra,  but  not  infrequently  occurs  as  irregular 
aggregates.  It  has  a  very  uniform,  brass-yellow  colour. 
Hardness  =  6  to  6- 5  ;  specific  gravity  =  4-9  to  5-2  ;  streak  =  black. 
Before  the  blowpipe  pyrite  gives  off  sulphur,  burning  with  a 
blue  flame.  It  is  decomposed  by  nitric  acid.  The  only 
minerals  with  which  pyrite  might  possibly  be  confounded  are 
chalcopyrite  (an  ore  of  copper),  magnetic  pyrite,  and  perhaps 
gold.  Gold,  however,  is  malleable,  and  the  others  are  not. 
Pyrite  is  paler  and  considerably  harder  (6  to  6-5)  than 


28  STRUCTURAL  AND  FIELD  GEOLOGY 

chalcopyrite  (3-5  to  4) — -the  streak  of  the  former  being  black, 
while  that  of  the  latter  is  greenish-black.  Magnetic  Pyrite  or 
Pyrrhotite  (an  iron-sulphide  of  variable  composition)  has  a 
characteristic  pinchbeck-bronze  colour,  is  slightly  magnetic, 
and  not  so  hard  as  pyrite,  while  the  streak  is  greyish-black. 
TPyrite  often  occurs  in  the  form  of  detached  crystals  and 
aggregates  in  clay-slate.  It  is  an  occasional  ingredient  of 
schistose  rocks,  sandstone,  coals,  and  argillaceous  rocks  of 
various  kinds,  often  as  fine-grained  impregnations.  Now 
and  again  it  appears  as  an  accessory  mineral  in  eruptive 
rocks.  It  is  of  frequent  occurrence  also  in  lodes,  either  as 
crystal  aggregates  or  massive.  Pyrrhotite  is  not  so  common  a 
rock-former  as  pyrite.  Occasionally  it  is  present  in  basic 
igneous  rocks  (gabbro,  basalt,  etc.)  and  schists  (amphibole 
rocks).  Like  pyrite,  it  often  occurs  in  metalliferous  veins — the 
two  minerals  being  not  infrequently  associated  in  the  so-called 
"  bedded  veins  "  or  "  quasi-bedded  ore  formations." 

Marcasite  is  an  orthorhombic  mineral,  having  the  same  composition 
as  pyrite.  It  occurs  usually  compact  or  cryptocrystalline,  and  is  often 
disseminated  in  minute  grains  through  certain  sedimentary  rocks. 
Radiated  nodular  forms  are  also  very  common.  The  hardness  is  the 
same  as  that  of  pyrite,  and  the  specific  gravity  slightly  less.  It  is  a 
less  stable  form  than  pyrite.  The  colour  is  bronze-yellow,  inclined  often 
to  green  or  grey.  It  has  hardly  so  wide  a  distribution  as  pyrite,  occurring 
chiefly  as  concretions  in  argillaceous  and  calcareous  rocks. 

V.  CARBONATES 

Calcite  or  Calc-spar  (calcium  carbonate)  crystallises  in  the 
hexagonal  system,  and  assumes  a  great  variety  of  crystalline 
forms.  The  cleavage  is  rhombohedral,  as  exemplified  by  the 
well-known  transparent  Iceland  spar,  so  commonly  used  for 
polarising  instruments ;  but  the  unit  rhombohedron  is  a  rare 
crystal.  Scalenohedral  forms  are  very  common,  as  in  dog- 
tooth spar.  Calcite  is  recognised  by  its  slight  hardness  (  =  3), 
as  it  is  easily  scratched  with  the  penknife,  by  the  readiness 
with  which  it  effervesces  briskly  with  dilute  hydrochloric  acid, 
and  by  its  marked  rhombohedral  cleavage.  The  specific 
gravity  is  2-6  to  2-8.  Calcite  is  an  important  constituent  of 
many  aqueous  deposits — as  limestone,  marble  (Plate  IX.  2), 
calc-sinter,  etc.  It  is  a  frequent  binding  material  in  sedimen- 


HOCK-FO&MING  MINERALS  29 

tary  rocks.  As  a  secondary  product,  it  appears  commonly  in 
the  minute  pores  and  capillaries  of  many  different  minerals 
and  rocks;  it  also  occupies  cracks,  fissures  (see  Plate  XV.), 
and  cavities  of  all  kinds — being  a  common  gangue-mineral  in 
lodes.  It  is  the  chief  petrifying  agent,  and,  next  to  quartz, 
the  commonest  of  all  minerals. 

Aragonite  has  the  same  composition  as  calcite,  but  crystallises  in 
the  orthorhombic  system.  Its  hardness  (3-5  to  4)  and  specific  gravity 
(2-9  to  3)  are  both  somewhat  greater  than  those  of  calcite.  It  is  a  more 
soluble  form  of  calcium-carbonate  than  calcite,  and  not  nearly  so 
common  as  that  mineral.  Sometimes  it  is  met  with  in  beds  associated 
with  gypsum  and  iron-ore,  and  not  infrequently  in  cracks  and  cavities 
^in  recent  eruptive  rocks.  It  is  often  a  deposition  from  hot-springs. 

Dolomite  or  Bitter  Spar  (calcium  and  magnesium  carbonate)  crystal- 
lises in  the  hexagonal  system — the  faces  of  the  crystals  being  frequently 
curved.  Hardness  =  3-5  to  4-5  ;  specific  gravity  =  2-85  to  2-95.  It  is 
only  slightly  affected  by  cold  dilute  hydrochloric  acid,  but  is  dissolved 
when  the  acid  is  heated.  It  may  be  variously  coloured,  but  white  and 
yellow  varieties  are  most  common.  Magnesian  limestone  is  composed 
in  large  part  of  this  mineral. 

Siderite  or  Chalybite  (carbonate  of  iron)  occurs  usually  in  rhombo- 
hedral  forms,  often  with  curved  faces.  It  is  colourless  or  pale  yellow 
when  freshly  exposed,  but  soon  becomes  tarnished  brown  or  rusty.  Hard- 
ness =  3-5  to  4-5  ;  specific  gravity =  37  to  3-9  ;  the  mineral  is  infusible 
before  the  blowpipe,  but  effervesces  with  weak  acids.  It  occurs  in 
lodes  along  with  various  ores.  Spharosiderite  is  the  name  given  to  a 
compact  siderite  often  showing  a  concentric,  radiating,  fibrous  structure. 
It  occurs  as  nodules  and  nodular  masses  in  veins  and  cavities  in 
crystalline  schists,  etc.  Clay-ironstone  is  an  impure  variety  of  sphaero- 
siderite  mixed  with  clay,  which  occurs  as  nodules,  bands,  and  beds  in 
various  geological  formations.  Blackband-ironstone  is  a  clay-ironstone 
containing  a  notable  amount  of  carbonaceous  matter.  [Clay-ironstone 
and  blackband-ironstone  are  rather  rocks  than  minerals.] 

VI.  SULPHATES 

Anhydrite  (calcium  sulphate)  crystallises  in  the  orthorhombic  system, 
but  usually  occurs  massive  or  in  granular  and  fibrous  aggregates.  It 
is  often  associated  with  rock-salt  and  gypsum.  Hardness  =  3  to  3-5  ; 
specific  gravity  =  2-9  to  3.  It  is  slightly  soluble  in  hydrochloric  acid,  and 
fuses  before  the  blowpipe  with  difficulty  to  a  white  enamel,  colouring  the 
flame  reddish-yellow. 

Gypsum     (hydrated     calcium     sulphate)    crystallises    in 
monoclinic  forms,  its  hardness  (1-5  to  2)  and  specific  gravity 


30  STRUCTURAL  AND  FIELD  GEOLOGY 

(2-2  to  2-4)  being  considerably  less  than  those  of  anhydrite. 
It  may  be  variously  coloured,  but  is  usually  transparent  or 
white.  It  is  soluble  in  hydrochloric  acid.  Before  the  blow- 
pipe it  becomes  opaque  or  white,  exfoliates,  and  fuses  to  a 
white  enamel.  Crystals,  lenticular  concretions,  and  inter- 
rupted layers  of  gypsum  often  occur  in  clays.  Frequently  it 
appears  as  granular  and  compact  masses,  arranged  as  layers 
and  thick  beds  in  argillaceous  strata,  where  it  is  commonly 
associated  with  rock-salt  and  anhydrite.  Now  and  again  it 
forms  the  cement  or  binding  material  of  sandstone.  Selenite 
is  the  name  given  to  crystallised  gypsum ;  it  shows  perfect 
cleavage — the  laminae  being  flexible  but  not  elastic.  The 
very  fine-grained  cryptocrystalline  kinds  are  usually  termed 
Alabaster,  and  the  fibrous  varieties  Satin  Spar. 

Barytes  or  Heavy  Spar  (barium  sulphate)  crystallises  in 
the  orthorhombic  system.  Fibrous  varieties  are  common.  It 
is  not,  properly  speaking,  a  rock-former,  but  is  usually  met 
with  as  a  secondary  mineral  in  veins  and  other  cavities.  It 
is  commonly  associated  with  ores  (especially  sulphides)  in 
lodes.  Its  hardness  (3  to  3-5)  slightly  exceeds  that  of  calcite, 
but  its  greater  specific  gravity  (4-3  to  4-6)  and  its  resistance 
to  acids  at  once  distinguish  it  from  the  latter.  Barytes 
decrepitates  and  fuses  with  great  difficulty  before  the  blow- 
pipe, colouring  the  flame  yellowish-green. 

VII.  PHOSPHATES 

Apatite  (phosphate  of  lime,  containing  either  fluorine  or 
chlorine  :  hence,  chemically,  two  kinds  are  recognised — fluor- 
apatite  and  chlor-apatite).  This  mineral  crystallises  in  the 
hexagonal  system,  usually  as  six-sided  prisms.  Hardness  = 
5  ;  specific  gravity  =  3- 17  to  3-23.  It  is  soluble  in  hydrochloric 
acid,  and  fusible  with  difficulty  before  the  blowpipe.  It 
occurs  as  a  frequent  but  usually  a  microscopic  accessory 
ingredient  of  very  many  eruptive  rocks  and  crystalline  schists, 
commonly  in  the  form  of  long,  slender,  hexagonal  prisms  or 
needles.  It  is  a  frequent  inclusion  in  all  the  essential  con- 
stituents of  eruptive  rocks.  Next  to  magnetite,  it  has  the 
widest  distribution  of  all  accessory  rock-constituents.  Fine 
crystals  occur  in  the  drusy  cavities  of  some  granites,  as  like- 


ROCK-FORMING  MINERALS  3.1 

wise  in  gneisses.  It  is  met  with  also  as  irregular  layers  (often 
associated  with  magnetite)  among  schistose  rocks;  while 
crystals,  large  and  small,  not  infrequently  appear  in  talc-  and 
chlorite-schists,  and-  in  metamorphosed  limestones.  Again,  it 
forms  independent  veins  of  large  size,  associated  with  gabbro. 
The  earthy  and  concretionary  varieties  of  phosphate  of  lime 
are  known  as  Phosphorite — and  many  of  these  are  of  organic 
origin. 

VIII.  ELEMENTS 

Carbon,  in  the  form  of  Graphite,  is  the  only  element  which  plays  a 
relatively  considerable  part  as  a  rock-former.  Graphite  is  usually  not 
crystallised,  but  sometimes  it  appears  as  flat,  six-sided  plates.  Hardness 
=  i  ;  specific  gravity  =  2.  It  is  black,  with  an  almost  metallic  lustre; 
has  a  greasy  feel ;  and  yields  a  black  and  shining  streak.  It  is  not 
affected  by  acids.  It  occurs  as  a  constituent  (sparingly  or  abundantly,  as 
the  case  may  be)  of  many  schistose  rocks  and  slates,  as  in  graphite-schist, 
graphite-gneiss.  Now  and  again  lenticular  beds  of  it  appear  among 
schists,  and  not  infrequently  it  occupies  veins  and  other  cavities  travers- 
ing such  rocks.  It  has  been  met  with  also  in  granite  and  basalt.  Coal 
is  sometimes  converted  into  graphite  by  contact  with  eruptive  rock,  as  at 
New  Cumnock  and  near  Shotts,  in  Scotland. 

Many  minerals  and  rocks  are  rendered  dark  or  even  black  owing  to 
the  quantity  of  carbonaceous  matter  they  contain.  When  the  carbon- 
aceous matter  is  quite  amorphous  (i.e.  destitute  of  crystalline  form  and 
structure)  it  is  readily  driven  off  by  heating.  (Pure  graphite,  however, 
burns  only  with  the  greatest  difficulty  before  the  blowpipe.)  The  amor- 
phous carbonaceous  colouring  matter  of  black  marble,  etc.,  is  apt  to 
become  oxidised  on  exposure  to  the  weather,  and  changed  into  carbon- 
dioxide,  so  that  the  rock  tends  to  bleach  and  whiten. 


CHAPTER    III 

ROCKS 

Classification  : — Crystalline  Igneous  Rocks — their  general  characters. 
Chief  Minerals  of  Igneous  Rocks.  Primary  and  Secondary  Minerals. 
Law.  of  Mineral  Combination.  Groups  of  Igneous  Rocks  : — Rocks 
with  Dominant  Alkali  Felspar  ;  Rocks  with  Dominant  Soda-Lime 
Felspar ;  Rocks  with  Felspathoids  in  place  of  Felspars  ;  Rocks 
without  Felspars  or  Felspathoids  ;  Pyroclastic  Rocks. 

THE  term  "  rock,"  as  used  by  the  geologist,  means  any 
mass  or  aggregate  of  one  or  more  kinds  of  mineral  or  of 
organic  matter,  whether  hard  and  consolidated  or  soft  and 
incoherent,  which  owes  its  origin  to  the  operation  of  natural 
causes.  Thus  granite,  basalt,  limestone,  clay,  sand,  silt,  and 
peat,  are  all  equally  termed  rocks. 

Speaking  generally,  we  may  say  that  the  unconsolidated 
rocks  occupy  for  the  most  part  a  superficial  position — over- 
spreading and  concealing  the  consolidated  rocks  of  which  the 
earth's  crust  is  chiefly  composed.  There  are  many  exceptions 
to  this  rule,  however.  Sometimes,  for  example,  unconsolidated 
materials  occur  at  considerable  depths  from  the  surface,  buried 
under  masses  of  hard  rock.  Nor  is  the  relative  age  of  a  rock 
always  indicated  by  the  degree  of  its  consolidation.  Many 
incoherent  rocks  are  of  great  geological  antiquity ;  while,  on 
the  other  hand,  some  rocks  of  quite  recent  age  are  never- 
theless as  hard  and  resistant  as  the  oldest. 

Classification  of  Rocks. — The  rocks  of  which  the  earth's 
crust  is  constructed  are  very  diverse  in  character  and  origin. 
Some  owe  their  origin  to  eruptive  and  volcanic  forces  ;  others 
are  obviously  composed  of  materials  which  have  been  derived 
from  the  disintegration  of  pre-existing  rock-masses ;  while 
yet  others  have  undergone  certain  more  or  less  fundamental 

32 


MICROSCOPIC  STRUCTURE  OF  MINERALS  AND  ROCKS. 


2. 


1.  Banded  Obsidian. 

2.  Fluxion  structure  in  Pitchstone. 

3.  Pegmatitic  structure :  intergrowth  .of  Quartz  and  Felspar.    Graphic  Granite. 

4.  Ophitic  structure :  lath-shaped  crystals  of  Felspar  enclosed  in  a  large  plate  of  Augite,  which 

shows  parallel  cleavage.    Diabase. 


MICROSCOPIC  STRUCTURE  OF  MINERALS  AND  ROCKS. 


1.  Granophyric   structure:    minute   intergrcnvth   of  Quartz  and   Felspar.      Grauophyre.      Nicols 

crossed. 

2.  Calcite,  with  rhombohedral  twinning  and  cleavage.    Marble.     Nicols  crossed. 

3.  Trachytic  structure  :  small  crystals  of  Felspar  in  fluxional  arrangement.     Bostonite.     Nicols 

crossed. 

4.  Trachytic  structure  :    microlites  of  Felspar  eddying  round  a  group  of  larger  crystals  of  Sani- 

dine.    Trachyte.     Nicols  crossed. 


[To  face  pa 


ROCKS  33 

changes  since  the  time  of  their  formation,  so  that  it  is  not 
always  possible  to  tell  what  their  original  character  may  have 
been.  We  have  thus  three  more  or  less  well-marked  types  of 
rocks,  which  may  be  termed  Igneous,  Derivative,  and  Meta- 
morphic  rocks  respectively. 

I.   IGNEOUS   ROCKS 

This  division  includes  all  masses  which  owe  their  origin 
to  the  operation  of  eruptive  and  volcanic  forces.  Some  of 
these  rocks  consist  either  wholly  or  in  part  of  crystalline 
ingredients,  while  others  are  composed  of  fragmental  materials. 
Hence  we  have  two  groups,  viz. : — A.  Crystalline,  and  B. 
Fragmental  or  Clastic  Igneous  Rocks. 

A.  Crystalline  Igneous  Rocks 

The  rocks  of  this  group  vary  much  in  character.  Some 
are  thoroughly  crystalline,  while  others  consist  partly  of 
crystalline  minerals  and  partly  of  non-differentiated  matter — 
the  relative  proportion  of  crystalline  and  non-crystalline 
ingredients  varying  indefinitely. 

All  these  rocks  have  consolidated  from  a  state  of  igneous 
fusion — the  general  character  of  each  having  been  largely 
determined  by  the  conditions  under  which  the  original  molten 
matter  or  magma  has  cooled  and  solidified.  That  magma 
has  a  complex  chemical  composition,  but  may  be  said  to 
consist  essentially  of  a  mixture  of  several  silicates  and 
oxides,  with  water  and  various  gases.  As  soon  as  the 
temperature  of  this  mixture  begins  to  fall,  the  commingled 
ingredients  commence  to  separate  out  successively — in  other 
words,  molecules  of  a  like  kind  gather  and  group  them- 
selves together  to  form  crystals.  Sometimes  the  cooling 
process  is  so  protracted  that  all  the  several  compounds 
constituting  the  magma  have  time  to  become  thoroughly 
crystallised.  In  other  cases  solidification  takes  place  more 
rapidly,  so  that  crystallisation  is  only  partially  effected,  and 
the  resulting  rock  then  consists  of  a  mixture  of  crystalline 
ingredients  and  glassy  or  non-differentiated  matter.  Occa- 
sionally, indeed,  cooling  and  consolidation  proceed  so  promptly 
that  no  crystals  have  time  to  form  before  the  whole  mass 

C 


34  STRUCTURAL  AND  FIELD  GEOLOGY 

congeals  to  form  a  vitreous  rock,  throughout  which  the 
several  mineral  compounds  exist  in  essentially  the  same 
diffused  condition  as  in  the  molten  magma.  It  will  be 
understood,  then,  that  when  a  molten  mass  cools  and  con- 
solidates rapidly,  a  glassy  or  vitreous  rock  results ;  with  less 
rapid  cooling  a  hemicrystalline  rock  is  formed ;  while  very 
slow  cooling  gives  rise  to  a  holocrystalline  rock.  But  as 
traces  of  crystallisation  are  rarely  or  never  quite  absent 
from  a  volcanic  glass,  all  these  types  may,  for  purposes  of 
description,  be  included  under  the  head  of  crystalline  igneous 
rocks. 


Vitreous  Rocks.  Their  General  Character. — Many  of  these  seem  to  the 
Unassisted  eye  smoothly  homogeneous,  and  to  contain  no  trace  of  crystal- 
line ingredients.  When  thin  slices,  however,  are  subjected  to  microscopic 
examination,  they  rarely  fail  to  show,  in  less  or  greater  abundance, 
certain  minute  bodies,  some  of  which  have  obviously  a  crystalline 
structure,  while  others  show  no  such  structure,  but  may  be  looked  upon 
as  merely  the  embryos  of  crystals.  Some  of  these  forms  are  shown  in 
Plate  VII.  i.  Crystallite  is  the  name  given  to  the  minute  bodies  which 
do  not  react  on  polarised  light,  and  are  apparently  destitute  of  crystalline 
structure.  As  a  rule  crystallites  afford  no  hint  as  to  the  nature  of  the 
mineral  into  which  they  might  have  developed  had  their  growth  not  been 
arrested.  Other  minute  bodies  (Microlites}  which  give  a  definite  reaction 
with  polarised  light,  show  a  further  stage  in  crystal-development,  and 
are  often  of  such  a  character  that  it  is  possible  to  say  to  what  mineral 
species  they  belong.  Besides  crystallites  and  microlites,  more  or  less 
well-developed  crystals  of  relatively  large  size  may  occur  disseminated 
through  a  vitreous  rock  (see  Plate  VII.  2). 

Certain  other  structures  of  frequent  occurrence  in  glassy  rocks  may  be 
briefly  referred  to.  Amongst  these  are  small  globules  termed  Spherulites 
(see  Plates  VII.  3  ;  XIII.  i).  They  vary  in  size  from  a  millet-seed  to  a 
pea,  and  under  the  microscope  show  an  internal  divergent  or  radiating 
fibrous  structure.  Similar  spherical  bodies,  sometimes  larger  than  hazel- 
nuts,  are  now  and  then  developed  in  artificial  glass,  their  internal  fibrous 
structure  being  quite  apparent  to  the  naked  eye.  Not  infrequently, 
glassy  rocks  contain  small  enamel-like  globules,  which,  in  thin  sections 
under  the  microscope,  often  exhibit  an  imperfectly  developed  concentric 
or  perlitic  structure  (see  Plate  VII.  4).  Spherulites  may  occur  sporadi- 
cally or  be  closely  packed  together  ;  and,  similarly,  perlitic  structure 
may  be  sparsely  or  abundantly  developed — some  glassy  rocks,  indeed, 
appearing  as  if  composed  entirely  of  enamel-like  globules.  A  vitreous 
rock  having  this  character  well  marked  is  often  termed  Perlite. 

There  are  certain  other  structures  which,  although  not  confined  to 
vitreous  rocks,  are  nevertheless  more  or  less  characteristic  of  these. 


ROCKS  35 

Frequently,  the  crystallites,  microlites,  crystals,  and  spherulites  contained 
in  a  glass  appear  arranged  in  lines  or  in  bands  ;  very  often,  too,  the  glass 
shows  a  ribboned  or  striped  appearance — darker  and  lighter  coloured 
layers  rudely  alternating  (see  Plates  VIII.  i,  2  ;  XIII.  2).  This  is  known 
as  Fluxion  or  Fluidal  structure,  and  is  obviously  due  to  the  differential 
movement  of  the  rock  while  it  was  still  in  a  mobile  condition. 

All  molten  masses  contain  water  and  various  'vapours  and  gases,  which 
are  given  off  in  dense  clouds  from  a  lava  at  the  time  of  its  eruption. 
When  the  lava  is  very  liquid  the  steam  readily  escapes  ;  but  as  the  mass 
on  cooling  becomes  more  viscous  the  vapours  are  less  easily  got  rid  of. 
They  segregate  and  expand,  pushing  the  plastic  rock  aside  and  thus 
forming  spherical  cavities.  In  this  way  the  upper  portion  of  a  lava  is 
often  rendered  more  or  less  vesicular.  As  the  lava  flows  on  its  way 
the  spherical  cavities  become  flattened  and  drawn  out  in  the  direction  of 
movement.  The  vesicles  vary  in  size  from  mere  pores  up  to  cavities, 
measuring  more  than  one  foot  across  ;  but  cavities  of  such  a  size  occur 
only  sporadically.  In  the  case  of  vitreous  rocks  which  have  flowed  out 
in  a  highly  liquid  condition,  the  vesicles  are  rarely  large.  Usually  they 
are  so  small  and  so  very  abundant  that  they  may  occupy  fully  as  much 
space  as  the  solid  portion  which  contains  them.  Vitreous  rock  of  this 
kind  has  a  spongy,  froth-like  appearance,  and  is  known  as  Pumice.  The 
vesicles  formed  in  very  viscous  lavas  are  usually  larger  and  not  so 
abundant. 

Hemicrystalline  Rocks.  Their  General  Character. — These  rocks  are 
composed  chiefly  of  crystalline  ingredients,  with  a  larger  or  smaller 
proportion  of  non-differentiated  matter.  Typically,  a  hemicrystalline  rock 
contains  the  following  constituents  : — (a)  Groundmass,  an  aggregate  of 
microlites  and  small  crystals  or  crystalline  granules,  with  which 
some  amount  of  glass  (not  infrequently  devitrified)  may  or  may  not  be 
associated ;  (b]  Phenocrysts^  the  term  applied  to  the  larger  crystals 
disseminated  through  the  groundmass. 

Most  hemicrystalline  rocks  have  consolidated  at  or  near  the  surface 
of  the  earth.  While  they  were  still  in  a  molten  condition,  however,  and  at 
some  considerable  depth  in  the  crust,  cooling  had  already  commenced, 
and  certain  minerals  had  crystallised  out.  Such  minerals,  therefore, 
eing  free  to  develop,  often  attained  a  relatively  large  size  and  a  more 
or  less  perfect  crystalline  form.  Not  infrequently,  however,  they  show 
corroded  outlines,  as  if  they  had  been  partially  dissolved.  This  is 
supposed  to  have  been  caused  by  the  action  of  the  still  fluid  portion  of 
the  magma — rendered  more  acid  as  it  would  be  after  the  phenocrysts 
had  separated  out.  Probably  the  process  of  resorption  was  aided  also 
by  changes  of  pressure  and  temperature  as  the  molten  rock  rose  towards 
the  surface.  Not  only  are  the  phenocrysts  frequently  corroded,  but  they 
have  often  been  broken  during  movements  of  the  magma.  Thus,  when  a 
molten  mass  eventually  reached  the  surface,  it  already  contained  many 
disseminated  solid  particles — the  phenocrysts.  No  sooner  did  the  lava 
begin  to  flow  than  cooling  proceeded  so  rapidly  that  large  and 
approximately  perfect  crystals  could  no  longer  be  formed— the  numerous 


36  STRUCTURAL  AND  FIELD  GEOLOGY 

mineral  bodies  interfering  with  each  other's  growth  and  thus  forming  a 
close  aggregate,  diffused  through  which  glassy  matter  might  occur  either 
sparingly  or  abundantly.  This  is  the  so-called  groundmass.  When 
phenocrysts  are  conspicuously  present  in  it  we  have  what  is  known  as 
porphyritic  structure  (see  Plates  X.  ;  XII.  i).  It  will  be  understood, 
therefore,  that  such  porphyritic  rocks  give  evidence  of  two  stages  of  con- 
solidation— the  phenocrysts  belonging  to  the  earlier  or  intratelhiric,  and 
the  groundmass  to  the  final  or  volcanic  stage. 

The  groundmass  of  hemicrystalline  rocks  is  as  a  rule  mostly  made 
up  of  crystalline  ingredients.  In  some  of  these  rocks,  however,  it 
consists  chiefly  of  glass,  while  in  many  others  crystalline  constituents  and 
glass  are  approximately  equal  in  amount.  The  non-differentiated  matter 
or  base  of  the  groundmass  not  infrequently  assumes  a  microfelsitic 
or  cryptocrystalline  character.  To  the  unassisted  eye  this  substance 
seems  to  be  quite  compact  and  homogeneous  ;  but  under  the  micro- 
scope microfelsitic  matter  appears  as  an  indefinite  aggregate,  or  nearly 
structureless  mass. 

As  might  have  been  expected,  the  non-differentiated  matter  in  the 
groundmass  frequently  shows  the  structures  which  have  already  been 
described  as  characteristic  of  volcanic  glass.  It  is  often  more  or  less 
devitrified  and  stony-like,  owing  to  the  abundant  development  of 
crystallites  and  microlites,  while  spherulitic  and  perlitic  structures  are  of 
common  occurrence.  Fluxion  structure  also  is  often  seen,  not  only  in  the 
base  but  throughout  the  whole  groundmass.  Lastly,  vesicular  structure, 
as  already  indicated,  is  just  as  characteristic  of  hemicrystalline  as  of 
vitreous  rocks. 

Holocrystalline  Rocks.  Their  General  Character. — These  rocks  con- 
tain no  non-differentiated  matter — they  have  no  base,  and  as  a  rule  no 
proper  groundmass  (see  Plate  XII.  2).  Not  infrequently,  however,  they 
show  conspicuous  phenocrysts  disseminated  through  the  relatively  fine- 
grained crystalline  aggregate  which  constitutes  the  mass  of  the  rock. 
Such  rocks  would  therefore  seem  to  have  experienced  two  stages  of  solidi- 
fication— the  phenocrysts,  as  usual,  having  crystallised  out  first.  Both 
stages  of  solidification,  however,  were  intratelluric  —  holocrystalline 
rocks  being  usually  of  more  or  less  deep-seated  origin.  They  differ 
greatly  in  texture,  some  being  very  finely  crystalline,  while  others  are 
exceedingly  coarse-grained,  and  between  these  extremes  all  intermediate 
textures  occur. 

Mineral  Ingredients  of  Igneous  Rocks. — Many  different  minerals  enter 
into  the  composition  of  igneous  rocks,  anhydrous  silicates  being  by  far 
the  most  important.  Save  in  the  case  of  well-developed  phenocrysts 
and  the  smaller  accessory  ingredients,  these  minerals  are  not  as  a  rule 
completely  bounded  by  crystal  faces.  When  they  are  thus  bounded  they 
are  said  to  be  euhedral  or  idiomorphic.  Should  only  some  of  the  crystal 
faces  appear  (and  this  is  very  often  the  case  with  those  minerals  which 
were  the  first  to  crystallise  out  after  the  phenocrysts  and  smaller 
accessories  had  appeared),  then  the  structure  is  termed  subhedral  or 
hypidiomorphic.  Most  commonly,  however,  the  minerals,  owing  to 


ROCKS  37 

mutual  interference,  have  not  assumed  the  geometrical  forms  which 
would  have  distinguished  them  had  they  crystallised  under  more 
favourable  conditions.  Minerals  thus  devoid  of  their  proper  crystalline 
form  are  described  as  anhedral  or  allotriomorphic — their  shape  has  been 
determined  by  their  surroundings. 

Inclusions  in  Minerals. — When  examined  in  thin  slices  under  the 
microscope,  the  minerals  of  igneous  rocks  are  often  seffn  to  include 
minute  crystals  or  crystalline  granules  of  other  minerals.  Not  infre- 
quently, also,  cavities,  containing  gas  or  liquid,  or,  it  may  be,  glass  or 
stony  matter,  appear  in  less  or  greater  abundance  (see  Plates  I.  2  ;  IV.  4  ; 
VIII.  4).  These  inclusions  are  termed  cndomorphs — the  minerals  which 
contain  them  being  termed  perimorphs.  Obviously,  all  these  foreign 
bodies  must  have  been  caught  up  and  enclosed  while  the  perimorphs 
were  separating  out  from  the  original  molten  magma. 

Primary  or  Original  Minerals. — Those  rock-constituents  which  crystal- 
lised out  from  the  magma  are  termed  primary  or  original,  to  distinguish 
them  from  another  group  of  minerals  which  are  of  later  origin  than  the 
rocks  in  which  they  occur.  Two  kinds  of  primary  minerals  are  recog- 
nised— namely,  (a)  Essential  and  (b]  Accessory  minerals.  Essential 
minerals  are  those  which  determine  the  species  of  a  rock,  while  accessory 
minerals  are,  as  it  were,  mere  accidental  ingredients,  the  presence  or 
absence  of  which  does  not  affect  the  general  character  of  a  rock. 
Granite,  for  example,  is  composed  of  three  essential  minerals — felspar, 
quartz,  and  mica.  Take  away  any  one  of  those,  and  the  rock  ceases  to  be 
a  granite.  One  or  more  non-essential  ingredients,  however,  may  be 
present,  and  yet  the  rock  remains  a  granite.  Should  one  of  these 
accessory  minerals  be  very  abundant  or  conspicuous,  it  may  give  rise  to  a 
variety.  If  a  granite,  for  example,  contains  conspicuous  crystals  of 
hornblende  or  of  tourmaline,  it  is  termed  a  hornblendic  or  a  tourmaline 
granite,  as  the  case  may  be. 

Secondary  Minerals. — All  rocks  are  subject  to  alteration,  due  especi- 
ally to  the  action  of  water  percolating  through  them.  This  water  finds 
its  way  along  fissures  and  other  planes  of  division,  and  soaks  into 
the  rock  itself  through  the  minute  cracks,  capillaries,  and  interstitial 
pores,  which  are  never  wanting  in  even  the  most  compact  and  homo- 
geneous kinds.  The  percolating  water  contains  carbon-dioxide  or  other 
acid  in  solution,  which  has  been  taken  up  from  the  atmosphere  by  rain, 
or  absorbed  from  the  soil.  Thus  armed,  the  water  attacks  the  various 
mineral  constituents  of  rocks,  which  in  this  way  may  be  more  or  less  pro- 
foundly altered.  Some  yield  much  more  readily  than  others,  but  sooner 
or  later  the  several  silicate  minerals,  of  which  igneous  rocks  are  so  largely 
composed,  tend  to  be  chemically  broken  up — such  bases  as  the  alkalies 
and  alkaline  earths  being  removed  in  solution  as  bicarbonates.  Some 
crystalline  igneous  rocks  have  been  so  much  affected  by  the  chemical 
action  of  water,  that  they  have  been  changed  from  hard,  resisting  masses, 
showing  a  sparkling  lustre  on  freshly  fractured  surfaces,  to  dull,  soft, 
earthy,  or  clay-like  substances,  which  may  be  dug  with  a  spade.  Few 
igneous  rocks,  indeed,  which  have  been  long  exposed  to  the  insidious 


38  STRUCTURAL  AND  FIELD  GEOLOGY 

action  of  percolating  water,  fail  to  show  some  trace  of  alteration.  They 
may  appear  to  be  fresh  to  the  unassisted  eye,  but  thin  slices  viewed  under 
the  microscope  will  almost  invariably  show  that  one  or  other  of  their 
mineral  constituents  has  undergone  change.  The  felspar  of  a  granite,  for 
example,  is  a  mineral  which,  when  unaltered,  appears  quite  clear  and 
transparent  in  thin  slices.  When  alteration  has  commenced,  this  is 
shown  by  a  clouded  or  turbid  aspect,^ affecting  the  whole  or  a  portion 
only  of  the  mineral.  Increasing  turbidness  marks  increasing  chemical 
alteration  ;  until  eventually  all  trace  of  the  original  felspar  disappears, 
and  its  place  is  occupied  by  a  white  or  greyish  homogeneous  substance. 
This  substance  is  the  hydrous  silicate  of  alumina,  known  as  kaolinite,  and 
is  obviously  the  result  of  the  complete  decomposition  of  the  felspar  which 
it  replaces.  As  felspar  is  an  anhydrous  silicate  of  alumina  and  alkali  or 
alkaline  earth,  the  chief  change  brought  about  has  been  the  removal  of 
the  soluble  bases — the  more  resisting  silicate  of  alumina  being  left  behind 
as  a  hydrate.  It  is  quite  common,  in  this  way,  for  certain  minerals  of 
igneous  rocks  to  become  changed  into  other  mineral  species,  either  by 
the  gain  or  loss  of  some  ingredient,  or  by  the  gain  of  one  ingredient  and 
the  loss  of  another.  The  new  mineral  thus  formed  is  known  as  an 
alteration-pseudomorph  ;*  and  all  such  products  of  alteration  are  termed 
secondary  minerals — they  are  thus  of  later  origin  than  the  rock  of  which 
they  form  a  portion. 

It  will  be  understood  now  that  secondary  minerals  are  simply 
the  products  of  the  chemical  alteration  of  essential  and  accessory 
minerals.  They  not  only  replace  in  whole  or  in  part  these  primary 
or  original  constituents,  but  are  frequently  met  with  lining  or  filling 
cracks  and  fissures,  or  occupying  the  vesicular  cavities  of  igneous 
rocks. 

Chief  Minerals  of  Igneous  Rocks. — A  large  number  of  minerals  enter 
into  the  composition  of  igneous  rocks — the  more  important  of  which 
have  been  described  in  preceding  chapters.  These,  as  we  have  seen, 
naturally  fall  into  two  groups  :  (i)  PRIMARY  or  ORIGINAL,  and  (2) 
SECONDARY  minerals.  The  former  group  includes  two  kinds,  namely, 
Essential  and  Accessory,  and  may  be  tabulated  as  follows  : — 

PRIMARY  OR  ORIGINAL  MINERALS 

In  list  I.  we  include  the  most  important,  namely,  those  which  have 
the  widest  distribution  and  occur  most  abundantly — those,  in  short, 
which  are  the  chief  ingredients  of  the  commonest  igneous  rocks.  The 
minerals  given  in  italics  are  of  less  importance  than  the  others.  All  the 

*  A  pseudomorph  is  simply  a  crystalline  or  amorphous  body  which 
has  assumed  the  crystalline  form  of  another  mineral.  There  are  several 
kinds  of  pseudomorphs.  In  certain  cases  a  mineral  may  be  dissolved  out 
of  a  rock  and  a  cavity  or  mould  left ;  subsequently  mineral  matter  of  a 
different  kind  may  be  introduced  by  infiltration  into  the  cavity,  in  which 
case  we  have  a  substitution-pseudomorph. 


ROCKS  39 

minerals  in  the  list  are  essential  constituents  in  some  rocks,  and  accessory 
ingredients  in  others  : — 

I. 

1.  Quartz.  5.  Biotite. 

2.  Felspars.  6.  Olivine. 

3.  Pyroxenes.  7.  Nepheline. 

4.  Amphiboles.  8.  Leucite. 

The  minerals  named  in  list  II.  are  of  less  importance — the  igneous 
rocks  of  which  they  are  essential  constituents  being  of  more  local 
occurrence.  As  accessory  ingredients,  however,  they  play  a  notable 
part,  some  of  them  (muscovite,  garnet,  schorl,  sphene)  having  a  very 
wide  range  indeed  : — 

II. 

1.  Muscovite.  4.  Garnet. 

2.  Sodalite.  5.  Schorl. 

3.  Haiiyne  and  Nosean.  6.  Sphene. 

The  minerals  in  list  III.  occur  chiefly  as  accessory  ingredients,  and 
are  thus  of  subordinate  importance  to  those  already  mentioned,  but  they 
are  all  very  widely  distributed  : — 

III. 

1.  Apatite.  5.  Pyrite. 

2.  Magnetite.  6.  Zircon. 

3.  Ilmenite.  7-  Rutile. 

4.  Haematite. 

List  IV.  includes  accessory  ingredients  which  are  not  so  widely  dis- 
tributed as  those  already  mentioned  : — 

IV. 

1.  Spinel.  3.  Picotite. 

2.  Chromite.  4.  Pyrrhotite. 

SECONDARY   MINERALS 

There  are  many  minerals  of  secondary  origin,  but  only  the  more 
commonly  occurring  ones  are  mentioned  in  the  following  list : — 

1.  Quartz,  opal,  chalcedony.  7.  Muscovite. 

2.  Calcite,  aragonite.  8.  Serpentine. 

3.  Zeolites.  9.  Epidote  (Pistazite). 

4.  Iron  oxides.  10.  Leucoxene. 

5.  Chlorite.  n.  Kaolin,  etc. 

6.  Talc. 

The  Law  of  Mineral  Combination.— The  more  important  original 
constituents  of  igneous  rocks  may  be  grouped  as  follows  : — 

1.  Felspathic  Silicates :  Felspars  and  Felspathoids. 

2.  Ferromagnesian  Silicates :  Pyroxene,  Amphibole,  Biotite,  Olivine. 

3.  Free  Silica :  Quartz. 


40  STRUCTURAL  AND  FIELD  GEOLOGY 

4.  Accessory  Minerals:  Magnetite,  Ilmenite,  Haematite,  Apatite, 
Rutile,  Zircon,  Sphene,  etc. 

The  members  of  these  several  groups  combine  according  to  a  some- 
what definite  plan,  which  may  be  termed  the  law  of  mineral  combination. 
Thus,  in  igneous  rocks,*  one  or  more  members  of  the  first  group  (Fels- 
pathic  Silicates]  are  associated  with  one  or  more  members  of  the  second 
group  (Ferromagnesian  Silicates],  With  these  are  more  sparingly 
associated  members  of  the  fourth  group  (Accessory  Minerals'] ;  while 
Free  Silica  may  or  may  not  be  present. 

Classification  of  Crystalline  Igneous  Rocks 

No  quite  satisfactory  classification  of  these  rocks  is  at 
present  possible.  From  the  chemical  point  of  view  they 
have  been  grouped  according  to  the  percentage  of  silica  they 
contain,  as  acid,  neutral  or  intermediate,  and  basic,  but  there 
are  so  many  gradations  from  the  one  type  into  the  other 
that  this  arrangement  breaks  down  when  we  come  to  apply 
it.  We  find,  for  example,  that  certain  rocks  of  the  same 
general  character,  and  which  obviously  constitute  a  family, 
are  under  this  chemical  classification  divided  instead  of  being 
grouped  together.  Some  andesites,  for  instance,  would  be 
termed  intermediate,  while  others  would  be  described  as  basic. 
Perhaps  a  nearer  approach  to  a  satisfactory  classification  is 
reached  by  taking  into  consideration  the  mineralogical  con- 
stitution of  the  rocks,  and  arranging  them  according  to  the 
character  of  their  dominant  ingredients.  This  arrange- 
ment does  not  in  effect  differ  much  from  that  which  is 
based  on  the  silica  percentage,  but  it  has  at  least  the 
negative  merit  of  not  separating  closely  allied  types  of 
rock. 

The  most  important  rock-forming  minerals  are  un- 
doubtedly the  felspars.  In  the  great  majority  of  eruptive 
rocks  they  play  a  prominent  role,  since  a  large  number 
have  alkali  felspar  as  their  chief  constituent,  while  another 
considerable  division  is  characterised  by  the  presence  of  soda- 
lime  felspar  as  the  dominant  ingredient.  In  the  remaining 
types  of  rock,  felspar  is  either  absent  or  plays  the  subordinate 
part  of  an  occasional  accessory  mineral.  In  one  division 
of  those  rocks,  felspathoids  (nepheline,  leucite,  etc.)  take  the 

*  With  the  exception,  of  course,  of  the  ultra-basic  rocks,  which  contain 
neither  felspars  nor  felspathoids, 


PLATE  X.   L 


PtATE  XI. 


i.  GRAPHIC  GRANITE.    About  natural  size. 


2.  DRUSE  OR  GEODE  IN  GRANITE.    Nearly  natural  size. 


[Between  pages  40  and ,41. 


ROCKS  41 

place   of  felspar,   while   in    the  other   neither    felspars    nor 
felspathoids  are  present. 

i.  ROCKS  WITH  DOMINANT  ALKALI  FELSPAR 

This  group  includes  the  granites,  quartz-porphyries,  and 
rhyolites — all  acid  rocks  with  a  percentage  of  silica  ranging 
up  to  So,  and  the  syenites,  trachytes,  and  phonolites — 
intermediate  rocks  with  a  silica  percentage  ranging  up  to 
70  or  thereabout.  They  exhibit  all  kinds  and  degrees  of 
texture  and  structure,  some  being  crystalline,  others  hemi- 
crystalline,  and  yet  others  essentially  vitreous.  As  a  rule, 
the  coarse-grained  holocrystalline  types  are  of  deep-seated 
origin,  while  the  finer  grained  microgranitic  and  porphyritic 
types  are  usually  hypabyssal.  Many  of  these  finer  grained 
holocrystalline  rocks  occur  as  dykes,  veins,  and  sills.  The 
hemicrystalline  and  vitreous  types  have  as  a  rule  flowed  out 
as  lavas,  or  have  consolidated  as  intrusive  rocks  not  far  from 
the  surface. 

Granite  is  a  holocrystalline  aggregate  of  quartz,  alkali 
felspar  (orthoclase  and  microcline,  usually  accompanied  by 
some  plagioclase),  and  a  ferromagnesian  mineral  (mica  or 
hornblende),  the  constituents  occurring  as  crystalline  granules 
of  approximately  similar  size  (=  granitoid  structure).  The 
rock  varies  in  texture  from  microcrystalline  to  very  coarsely 
crystalline.  The  colour,  which  largely  depends  on  that  of  the 
felspar,  is  usually  light  or  dark  grey  or  reddish  ;  occasionally 
it  is  greenish.  In  coarse  or  medium  grained  granite  the 
essential  'minerals  are  readily  distinguished.  The  felspar 
appears  opaque,  and  is  sure  to  show  some  of  its  crystal  faces 
and  cleavage-planes  with  their  vitreous  or  pearly  lustre.  The 
quartz,  on  the  other  hand,  is  quite  irregularly  outlined.  It 
is  usually  dark  grey  but  transparent,  and  shows  a  kind  of 
glassy  lustre  on  its  uneven  broken  surfaces ;  there  is  no 
trace  of  cleavage.  The  mica  occurs  in  lustrous  plates  and 
scales  which  are  readily  separated  into  the  thinnest  lamellae. 
The  hornblende  is  recognised  by  its  dark  green  colour  and 
its  common  prismatic  or  columnar  appearance.  Accessory 
minerals  may  or  may  not  be  numerous,  some  of  the  com- 
monest being  apatite,  sphene,  zircon,  magnetite,  etc, 


42  STRUCTURAL  AND  FIELD  GEOLOGY 

Elf  f        •  -. 

A  few  varieties  of  granite  may  be  mentioned—only  the  essential 
minerals  being  named  : — Normal  or  Muscovite-granite  =*te\ spar  +  quartz 
+  muscovite  + biotite;  Granitite  or  Biotite-granite  (Plate  IV.  3)= felspar 
+  quartz  +  biotite  ;  Hornblende-granite  =  felspar  +  quartz  +  hornblende, 
and  usually  some  biotite  ;  Tourmaline  or  Schorl-granite  —  felspar  +  quartz 
+  schorl;  Graphic  granite  =  felspar  +  quartz,  which  have  crystallised 
together,  the  quartz  assuming  the  form  of  successive  irregular  columnar 
shells,  arranged  in  parallel  positions,  and  enclosed  in  the  felspar 
(see  Plate  XI.  i).  This  is  known  as  pegmatitic  structure  ;  when  seen  in 
cross-section  it  has  some  resemblance  to  Hebrew  writing,  hence  the 
name  graphic.  While  this  structure  often  occurs  megascopically, 
especially  in  coarsely  crystalline  veins  associated  with  granite,  it  is 
sometimes  only  to  be  detected  under  the  rmcro$co])z  =  micropegmatite 
(Plate  VIII.  3).  Giant-granite  or  Pegmatite  —  any  very  coarse-grained 
granite — granites  of  this  kind  very  frequently  show  pegmatitic  structure  ; 
Aplite  or  Haplite  —  2i  fine-grained  granite  containing  little  or  no  mica, 
met  with  as  veins  ;  Greisen  —  -a.  granite  with  little  or  no  felspar,  occurring 
as  veins  in  normal  granite  ;  Porphyritic  granite  =  a  rock  showing  large, 
phenocrysts*  of  felspar,  disseminated  through  a  relatively  fine-grained 
granitoid  matrix  ;  Granite-porphyry  or  Microgranite  =  &  rock  consisting 
of  a  microgranitic  or  micropegmatitic  (granophyric]  groundmass,  with 
phenocrysts  of  felspar,  quartz,  pyroxene,  and  occasionally  amphibole  ;  it 
occurs  sometimes  forming  a  part  of  a  large  mass  of  ordinary  granite  :  at 
other  times  it  forms  dykes  and  veins  proceeding  from  granite  ;  Granite- 
gneiss—  a  granite  in  which  the  minerals  have  in  whole  or  in  part  a  rudely 
parallel  arrangement,  giving  to  the  rock  a  coarsely  banded  structure. 

Structures  in  granite  : — Geodes  and  drusy  cavities  :  these  are  irregular 
cavities  which  often  occur  sporadically  in  granite,  and  are  usually  lined 
with  finely  crystallised,  well-formed  examples  of  the  essential  and  accessory 
ingredients  of  the  rock  (see  Plate  XL  2).  The  felspars  and  the  quartz  are 
generally  conspicuous,  and  with  these  mica  and  one  or  more  of  the 
accessory  ingredients,  as  sphene,  apatite,  zircon,  topaz,  beryl,  etc. 
Secretions  :  these  are  of  two  kinds — basic  and  acid.  The  basic  secretions 
are  dark  masses  of  very  irregular  form  and  varying  size,  rich  in  ferro- 
magnesian  minerals  (biotite,  hornblende),  sphene,  and  iron  ores.  They 
often  resemble  fragments  broken  from  some  other  rock,  and  subsequently 
enclosed  in  the  granite.  Possibly  they  may  be  fragments  of  massive 
aggregates  of  basic  ingredients  which  may  have  crystallised  out  from  the 
magma  at  an  early  stage  in  the  process  of  consolidation,  and  become 
broken  up  during  subsequent  movements  of  the  slowly  cooling  and 
consolidating  plutonic  mass.  The  acid  secretions  are  light-coloured, 

*  The  origin  of  such  phenocrysts  is  not  yet  understood.  The 
explanation  which  is  supposed  to  account  for  the  formation  of  phenocrysts 
in  lava-form  rocks  (see  supra,  p.  35)  can  hardly  apply  to  the  phenocrysts 
of  plutonic  rocks,  which  cooled  at  great  depths,  and  therefore  under  the 
continuous  pressure  of  heavy  overlying  rock  masses. 


PLATE  XII. 


i.  PORPHYRITIC  STRUCTURE. 


I.    FORPHYR1TIC   STRUCTURE,      y  UARTZ-PORPHYRY.      IN  atural  Size. 

^  JT  T&j*Wf  v* 
i^^2re?& 

fe.i^'f4^^j^5 


[To  face  page  42, 


ROCKS  43 

coarse  or  fine-grained  streaks  and  veins,  poor  in  ferromagnesian  con- 
stituents. These  veins  often  look  as  if  they  occupied  fissures  or  clefts. 
Such  fissures  may  be  supposed  to  have  originated  while  the  plutonic  mass 
was  only  in  part  solidified — the  still  mobile,  residual,  acid  magma  having 
been  squeezed  into  clefts  in  the  solidified  portions  during  movements  of 
the  gradually  cooling  rock.  The  granites  are  among  the  most  widely 
distributed  of  eruptive  rocks.  Although  most  are  of  deep-seated  origin, 
yet  a  few  appear  to  have  been  intruded  at  a  less  depth  from  the  surface. 

Quartz -porphyry. — This  is  a  hemicrystalline  rock.  The 
groundmass  is  sometimes  composed  of  a  microgranitic  aggre- 
gate of  quartz  and  felspar  (chiefly  orthoclase),  at  other  times 
it  is  cryptocrystalline.  Thus,  to  the  naked  eye  the  ground- 
mass  in  some  cases  appears  very  finely  crystalline,  and  in 
other  cases  dense  and  smoothly  compact.  Scattered  through 
the  groundmass  are  numerous  phenocrysts  of  quartz  and 
felspar  (either  or  both),  together  with  a  ferromagnesian 
mineral  (biotite  or  hornblende).  The  felspar  is  usually 
orthoclase,  but  plagioclase  (chiefly  oligoclase)  is  often 
associated  with  it  (see  Plate  XII.  i).  The  quartz  not  infre- 
quently occurs  in  the  form  of  corroded  bi-pyramidal 
crystals,  and  often  contains  inclusions  of  the  groundmass. 
The  colour  of  the  rock  depends  largely  upon  that  of  the 
orthoclase,  which  is  often  exceedingly  abundant,  and  may 
be  red  or  white.  Accessory  minerals  are  also  numerous, 
such  as  apatite,  sphene,  zircon,  magnetite,  etc.  As  the 
quartz  -  porphyries  are  mostly  very  old  rocks,  secondary 
minerals  are  generally  present,  especially  kaolinite,  chlorite, 
epidote,  and  muscovite.  Granophyre  is  a  quartz-porphyry, 
the']  groundmass  of  which  consists  of  a  micropegmatitic 
intergrowth  of  quartz  and  felspar  (see  Plate  IX.  i). 
The  quartz-porphyries,  like  the  granites,  are  very  widely 
distributed. 

Rhyolite  (Liparite  ;  Quartz-trachyte). — This  rock  is  usually 
somewhat  light-coloured  —  grey,  yellowish,  reddish,  or 
greenish.  It  is  hemicrystalline — the  groundmass  varying 
considerably  in  character  in  one  and  the  same  rock.  Some- 
times it  is  glassy  for  the  most  part,  or  it  may  be  crypto- 
crystalline, or  microcrystalline.  Scattered  throughout  are 
phenocrysts  of  sanidine,  plagioclase,  quartz,  biotite,  and 
occasionally  hornblende  or  augite. 


44  STRUCTURAL  AND  FIELD  GEOLOGY 

Under  the  microscope  what  seems  to  be  a  thoroughly  compact 
groundmass  is  sometimes  resolved  into  an  intimate  aggregate  of  lath- 
shaped  microlites  of  sanidine,  often  fluidally  arranged,  entangled  among 
which  are  frequently  seen  crystalline  granules  of  plagioclase,  quartz, 
zircon,  magnetite,  apatite,  etc.  In- other  cases,  however,  the  apparently 
compact  groundmass  is  found  to  be  composed  largely  of  glass  or  of 
cryptocrystalline  matter,  or  both,  and  usually  exhibits  perlitic,  spherulitic, 
and  fluxion  structures.  Occasionally  the  phenocrysts  are  so  large  and  so 
abundant  that  in  hand-specimens  little  or  no  groundmass  can  be  seen,  and 
the  rock  assumes  a  granitoid  aspect.  In  other  cases,  when  phenocrysts 
are  sparingly  present  or  wanting,  the  mass  has  a  porcellanous  or  enamel- 
like  appearance,  with  a  somewhat  waxy  lustre.  All  these  varieties  of 
texture  and  structure  may  occur  in  one  and  the  same  lava-flow — lenticular 
streaks,  laminae,  and  layers  of  coarser  and  finer  grained,  of  lighter  and 
darker  materials  alternating.  Frequently  the  rock  exhibits  a  finely 
porous  or  cellular  structure,  and  occasionally  spherical,  flattened,  and 
irregular  shaped  cavities  appear,  which  may  be  encrusted  or  filled  with 
quartz,  opal,  jasper,  chalcedony,  etc.  Probably  these  siliceous  minerals 
were  deposited  by  chemical  processes  before  the  rock  had  completely 
cooled  and  solidified.  Felsite  is  the  name  given  to  a  fine-grained  to 
compact  rock,  throughout  which  are  scattered  phenocrysts  of  orthoclase, 
plagioclase,  quartz,  and  ferromagnesian  minerals.  It  is  simply  a  more 
or  less  altered  rhyolite,  quartz-porphyry,  or  vitreous  rock.  Under  the 
microscope  it  shows  either  a  microcrystalline  or  cryptocrystalline  texture. 
Spherulitic  and  perlitic  structures  are  often  present. 

Rhyolites  have  a  somewhat  wide  distribution.  The  freshest  kinds  are 
of  Tertiary  age,  and  are  sparingly  represented  in  the  British  Islands  ;  they 
occur  chiefly  in  Ireland  (Antrim,  co.  Down)  ;  but  the  altered  kinds  are 
common  among  our  Palaeozoic  rocks,  occurring  both  lava-form  and 
intrusive. 

Pitchstone  and  Obsidian. — These  represent  the  vitreous 
condition  of  acid  rocks — they  are  hardly,  therefore,  indepen- 
dent rock-species,  for  they  very  often  occur  as  the  superficial 
crusts  of  hemicrystalline  acid  rocks.  They  contain  some 
73  per  cent,  of  silica.  Pitchstone  is  usually  dark  green  or 
black,  but  lighter  green,  red,  brown,  yellow,  and  even  white 
varieties  occur.  The  lustre  of  the  rock  is  pitch-like  or 
resinous ;  the  fracture  usually  conchoidal,  but  often  irregular 
or  splintery.  Sometimes  it  contains  very  few  crystallites  or 
microlites — at  other  times  it  is  crowded  with  such  inclusions 
—  the  microlites  occasionally  forming  skeleton-crystals, 
as  in  the  well-known  Arran  pitchstone,  where  they  are 
feather-like  and  dendritic  (see  Plate  VII.  i).  Phenocrysts 
now  and  again  abound ;  they  are  commonly  either  quartz 


[To  face  page  44. 


ROCKS  45 

(Plate  IV.  4)  or  an  acid  felspar  or  both;  green  augite  also  is 
often  present,  and  not  infrequently  biotite  or  hornblende ;  less 
common  are  rhombic  pyroxenes.  Pitchstone-porphyry  is  the 
name  given  to  this  rock,  when  the  phenocrysts  are  numerous 
and  prominent.  Obsidian  (see  Plate  XIII.)  is  grey  to  dark 
grey  and  black,  seldom  red  or  brown.  The  lustre  is  vitreous, 
and  the  fracture  conchoidal.  Phenocrysts  are  not  common — 
quartz  rarely  or  never  appearing.  Sometimes  this  glass  is 
crowded  with  crystallites,  spherulites,  microlites,  etc. ;  in  other 
cases  the  rock  is  almost  devoid  of  such  bodies. 

The  structures  characteristic  of  glassy  rocks  have  already  been 
described  (p.  34).  Perlite  is  a  glass  characterised  by  the  prevalence  of 
perlitic  structure,  just  as  Spherulite-rock  is  so  named  from  the  abundant 
development  of  spherulitic  structure.  Pumice  is  a  frothy,  foam-like, 
stringy,  cellular,  spongiform  acid  glass  :  it  does  not  form  individual  rock- 
masses,  but  occurs  as  a  crust  on  acid  lavas,  or  as  loose  blocks,  scoriae, 
cinders,  etc.  When  a  glassy  rock  becomes  crowded  with  crystallites, 
spherulites,  and  microlites,  it  acquires  a  stony  aspect,  and  is  said  to  be 
dcvitrificd. 

Obsidian  is  usually  associated  with  effusive  rhyolites — into  which 
indeed  it  frequently  passes.  It  occurs  in  Hungary,  the  Lipari  Islands, 
the  Canary  Islands,  Iceland,  the  Western  United  States,  Mexico, 
Ecuador,  New  Zealand,  etc.  Pitchstone  is  also  somewhat  widely 
distributed,  occurring  in  various  parts  of  Germany,  Tyrol,  N.  Italy, 
Scotland,  etc. ;  it  appears  commonly  in  the  form  of  dykes  and  intrusive 
sheets  or  sills. 

The  rocks  described  in  the  foregoing  pages  are  all  acid 
rocks,  having  a  similar  chemical  composition,  and  their 
different  and  often  strongly  contrasted  petrographical  aspect 
would  appear,  therefore,  to  be  due  to  the  varying  conditions 
under  which  they  cooled  and  solidified.  Rapid  cooling  of 
molten  matter,  as  we  have  seen,  results  in  the  production  of  a 
vitreous  rock,  while  protracted  cooling  gives  rise  to  a  hemi- 
crystalline  or  even  a  holocrystalline  type.  Geologists,  there- 
fore, look  upon  granite  as  the  deep-seated  equivalent  of  our 
acid  lavas,  or  rhyolites  and  rhyolite-glasses  or  obsidian.  But 
between  the  deep-seated  plutonic  granites  and  the  volcanic 
rhyolites,  occur  rocks  which  are  to  some  extent  intermediate 
in  character — that  is  to  say,  they  are  not  quite  so  crystalline 
as  granite,  and  not  usually  so  vitreous  as  the  rhyolites  and 
obsidians ;  these  are  the  quartz-porphyries.  Thus  the  same 


46  STRUCTURAL  AND  FIELD  GEOLOGY 

molten  matter,  if  it  were  poured  out  at  the  surface,  would 
solidify  as  an  obsidian  or  a  rhyolite ;  if  it  cooled  at  a  very 
considerable  depth  it  would  consolidate  into  granite ;  while  if 
it  were  injected  in  the  form  of  sills  and  dykes  at  a  less 
depth,  some  portions  might  become  microgranite  or  quartz- 
porphyry,  and  others,  which  had  cooled  more  rapidly,  pitch- 
stone. 

Syenite,  usually  reddish  but  not  infrequently  grey,  is  a 
holocrystalline  granitoid  aggregate  of  orthoclase  and  a  ferro- 
magnesian  mineral,  which  may  be  hornblende,  augite,  or 
mica.  Syenite  is  differentiated  from  granite  by  the  absence 
of  quartz.  Nevertheless,  under  the  microscope  a  little  quartz 
can  often  be  seen  straggling  among  the  other  ingredients. 
The  accessory  minerals  include  plagioclase,  which  is  rarely 
quite  wanting,  apatite,  zircon,  sphene,  ilmenite,  magnetite. 

Normal  Syenite  consists  essentially  of  orthoclase  and  hornblende. 
Augite-  or  Pyroxene -syenite  contains  orthoclase  and  plagioclase,  augite 
(sometimes  diallage),  hypersthene,  biotite,  and  a  little  quartz.  When  the 
chief  ferromagnesian  mineral  is  biotite,  we  have  the  variety  known  as 
Mica-syenite.  Elceolite-syenite  is  a  compound  of  alkali  felspar,  elaeolite, 
and  one  or  more  ferromagnesian  minerals  (pyroxene,  amphibole,  mica). 
The  rock  is  noted  for  the  variety  of  its  accessory  ingredients,  amongst 
which  are  plagioclase,  sphene,  apatite,  zircon,  fluor-spar,  sodalite,  and 
others. 

The  syenites  are  not  so  widely  distributed  as  the  granites.  The  type- 
rock  is  that  of  the  Plauenscher-grund,  Dresden.  Many  varieties  of 
syenite  occur  in  S.  Norway,  and  have  received  special  names  (Lauruikite^ 
Nordmarkite^  etc.).  Kentallenite  is  the  name  given  to  a  basic  syenite 
occurring  in  Argyllshire. 

Orthoclase-porphyry  (Orthophyre)  is  a  grey,  brown,  or  reddish  rock, 
the  groundmass  of  which  consists  essentially  of  microcrystalline  ortho- 
clase. Scattered  through  this  are  phenocrysts  of  orthoclase.  Plagioclase 
is  sometimes  present,  and  needles  of  hornblende,  scales  of  biotite,  or 
granules  of  augite,  may  often  be  observed  ;  a  little  quartz,  too,  occasionally 
appears.  The  most  conspicuous  ingredient,  however,  is  the  orthoclase. 

When  ferromagnesian  minerals,  such  as  biotite,  are  plentifully  present, 
this  rock  passes  over  into  Minette  or  Syenitic  Mica-trap.  This  rock,  when 
fresh,  is  dark  grey  to  black,  but  owing  to  weathering  it  is  often  brown. 
The  texture  is  medium  to  fine-grained  or  compact.  The  microscope 
shows,  however,  that  it  is  holocrystalline. 

Here  also  may  be  included  Bostonite^  a  light  yellowish  or  grey  rock, 
with  a  fine-grained  to  compact  groundmass,  composed  chiefly  of  small 
lath-like  crystals  of  felspar  (see  Plate  IX.  3).  Dark  ferromagnesian 
minerals  are  very  sparingly  present.  The  rock  occurs  in  dykes,  and  is 


ROCKS  47 

associated  with  alkali-granites,  alkali-syenites,  and  elaeolite-syenites. 
Orthoclase-porphyry  is  not  so  common  a  rock  as  quartz-porphyry.  It 
occurs  in  S.  Scotland,  where  it  is  associated  with  volcanic  rocks  of  Old 
Red  Sandstone  age. 

Trachyte  is  a  hemicrystalline  rock,  usually  light  or  dark 
grey  or  yellowish,  but  sometimes  brownish  or  even  reddish. 
The  texture  of  the  groundmass  is  commonly  close-grained, 
apparently  sometimes  compact ;  frequently,  however,  it  has  a 
rough,  porous  structure.  Disseminated  through  it,  pheno- 
crysts  are  usually  conspicuous,  especially  sanidine,  in  addition 
to  which  plagioclase,  hornblende,  biotite,  and  pyroxene 
frequently  occur. 

Under  the  microscope  the  groundmass  would  appear  to  consist 
essentially  of  lath-like  microlites  of  sanidine,  frequently  showing  fluxion 
structure,  and  often  entangling  a  few  granules  of  a  ferromagnesian 
mineral,  which  is  usually  augite  (see  Plate  IX.  4).  Some  interstitial 
glass  or  microfelsitic  matter  may  be  present.  Accessory  minerals  are 
numerous,  amongst  them  being  apatite,  magnetite,  zircon,  sphene  ;  while 
in  certain  trachytes,  sodalite  and  olivine  occasionally  make  their 
appearance. 

The  glassy  varieties  of  trachyte  are  known  as  Trachyte-obsidian  and 
Trachyte-pitchstone,  and  so  closely  resemble  the  rhyolitic  glasses  that 
they  can  hardly  be  distinguished  from  these  except  by  chemical  analyses. 
They  contain  a  lower  percentage  of  silica  (about  62). 

Trachyte  is  one  of  the  commonest  effusive  rocks  of  Tertiary  and  later  age 
— occurring  in  most  volcanic  districts  in  the  old  and  the  new  worlds. 
Trachytes,  however,  are  not  exclusively  young  rocks  ;  rocks  of  this  type 
occur  among  the  Old  Red  Sandstone  and  the  Carboniferous  volcanic 
series  of  Scotland. 

Phonolite  is  a  greenish  or  greyish  to  white  or  yellow,  and  sometimes 
brown  rock  composed  essentially  of  sanidine  and  nepheline  or  leucite 
(either  or  both).  The  texture  is  usually  compact,  with  a  somewhat  greasy 
lustre,  or  it  may  be  fine-grained  with  dull  lustre.  The  most  conspicuous 
phenocrysts  are  sanidine  and  nepheline  (or  leucite),  besides  which  the 
unassisted  eye  may  often  distinguish  pyroxene  or  amphibole.  The  rock 
is  characterised  by  the  absence  of  quartz,  by  its  somewhat  flaggy 
structure,  by- its  conspicuous  crystals  of  sanidine,  and  by  the  bell-like 
clink  it  gives  out  when  struck  with  the  hammer. 

The  microscope  proves  the  groundmass  to  consist  of  microlites  and 
small  crystals  of  sanidine  and  nepheline  (or  leucite),  which  not  infre- 
quently show  parallel  arrangement — a  structure  to  which  probably  the 
flaggy  structure  of  the  rock  is  in  some  measure  due.  Interstitial  glass 
is  rarely  present.  The  microscope  reveals  many  other  minerals  besides 
those  of  macroscopic  size,  such  as  the  common  accessories,  sphene, 
zircon,  etc.  ;  while  one  or  more  of  the  following  may  be  present :  biotite, 


48  STRUCTURAL  AND  FIELD  GEOLOGY 

amphibole,  pyroxene,  black  garnet,  sodalite,  haiiyne,  or  nosean,  etc. 
Leucite-phonolite  is  a  phonolite  in  which  leucite  takes  the  place  of 
nepheline  ;  when  haiiyne  occurs  instead  of  nepheline  we  have  Haiiyne- 
phonolite.  When  leucite  largely  takes  the  place  of  sanidine  the  rock  is 
known  as  Leticitophyre.  In  the  vacuoles,  fissures,  etc.,  of  all  phonolites, 
zeolites  and  calc.ite  are  of  common  occurrence. 

The  syenites  .play  much  the  same  part  as  the  granites  — 
occurring  as  more  or  less  deep-seated  plutonic  rocks  —  while 
orthoclase-porphyry  and  minette  are  also  intrusive,  but  appear 
chiefly  in  the  form  of  sills  and  dykes.  The  trachytes  may 
be  looked  upon  as  the  effusive  or  volcanic  representatives  of 
ordinary  syenites  ;  while  the  phonolites,  in  like  manner,  may 
be  the  effusive  equivalents  of  the  plutonic  elaeolite-syenites. 

2.  ROCKS  wiTtf  DOMINANT  SODA-LIME  FELSPAR 


This  group  includes  some  of  the  so-called  "  intermediate  " 
-types  (diorites  and  andesites),  and  others  which  are  on  the 
whole  more  basic  (gabbros,  dolerites,  and  basalts).  Like  the 
rocks  of  the  preceding  division,  they  are  holocrystalline,  hemi- 
crystalline,  and  vitreous. 

Diorite  is  a  holocrystalline  aggregate  of  plagioclase  and  a 
ferromagnesian  mineral  which  may  be  hornblende,  biotite, 
augite,  or  enstatite.  The  rock  varies  in  texture  from  granitoid 
to  compact  —  the  granitoid  varieties  being  speckled  green  and 
white,  while  the  compact  kinds  are  often  dark  green.  Acces- 
sory minerals  are  —  apatite,  magnetite,  sphene,  zircon,  etc. 

The  following  varieties  are  recognised  :  —  Quartz-diorite  =  quartz  + 
plagioclase  +  hornblende  or  biotite,  or  both.  [It  may  be  noted  that  a 
little  quartz  may  be  present  in  any  diorite.]  Mica-diorite  —  plagioclase 
+  biotite,  and  Augite-diorite  =  plagioclase  +  augite  —  in  both  these 
varieties  hornblende  is  often  present.  To  nalite  (Plate  XII.  2)  is  a  quartz- 
mica-diorite,  containing  some  orthoclase,  and  approaching  hornblende- 
granite.  Many  diorites  are  conspicuously  porphyritic.  Corsite  (Orbicular 
Diorite)  is  a  rock  in  which  the  constituents  have  crystallised  together  so 
as  to  form  spherical  aggregates  having  a  concentric  radiate  structure 
(Plate  XIV.).  Kersantite  is  a  dioritic  mica-trap  composed  essentially  of 
plagioclase  and  biotite.  The  diorites,  although  widely  distributed,  are 
not  very  abundant.  They  occur  chiefly  as  intrusive  masses  or  as  dykes 
and  veins. 

Andesite  is  a  hemicrystalline  rock,  usually  dark  coloured 
grey  to  brown.  It  consists  essentially  of  plagioclase  with  a 


i.  ORBICULAR  DIORITE  (CORSITE,  NAPOLEONITE).     Nearly  natural  size. 


2.     THE  SAME.     Natural  size. 


[To  face  page  48. 


[To  face  page  4 


ROCKS  49 

ferromagnesian  constituent — biotite,  augite,  hornblende,  or 
hypersthene — very  often  two  or  more  of  these  being  present. 
According  to  the  nature  of  the  predominant  ferromagnesian 
constituent,  the  rock  is  termed  Biotite-,  Hornblende- ,  Augite- , 
or  Hypersthene-andesite.  Many  andesites  are  more  or  less 
markedly  porphyritic  — the  phenocrysts  being  usually  plagio- 
clase,  and  one  or  more  of  the  ferromagnesian  minerals.  (See 
Plate  III.  3.) 

Under  the  microscope  the  groundmass  consists  of  lath-like  microlites 
and  crystals  of  plagioclase,  usually  fluidally  arranged,  with  minute  granules 
of  ferromagnesian  minerals,  magnetite,  etc.,  and  with  or  without  residual 
glass.  In  augite-  and  hypersthene-andesites  the  groundmass  is  often 
vitreous.  When  the  whole  rock  is  glassy  it  is  known  as  Andesitic  Obsidian 
or  Andesitic  Pitchstone.  Amongst  the  accessory  minerals  in  andesites  are 
apatite,  magnetite,  sanidine,  garnet,  sphene,  olivine,  zircon,  etc.  Dacite 
contains  quartz  in  addition  to  the  essential  ingredients  of  an  andesite. 
The  andesites  are  well  represented  in  the  Tertiary  and  later  volcanic 
districts  of  Europe  and  America.  More  or  less  altered  or  decayed 
andesites  are  very  common  in  the  British  Islands.  In  these  rocks 
(formerly  known  as  Porphyrites)  the  felspar  is  often  kaolinised,  while  the 
augite  and  hornblende  may  be  partially  or  wholly  converted  into  chlorite, 
the  hypersthene  changed  into  bastite  (a  fibrous  mineral  with  much  the 
same  composition  as  serpentine),  and  the  magnetite  into  haematite. 

Gabbro  is  a  granitoid  holocrystalline  aggregate  of  plagio- 
clase (basic  soda-lime  felspar)  and  a  ferromagnesian  silicate, 
the  texture  ranging  from  fine-grained  to  very  coarsely 
crystalline.  Several  varieties  are  recognised,  the  distinguish- 
ing character  of  each  being  the  particular  ferromagnesian 
constituent  that  happens  to  be  present.  Thus  normal 
Gabbro  =  plagioclase  +  diallage  ;  Norite  =  plagioclase  +  hyper- 
sthene ; '  Olivine-gabbro  and  Olivine-norite  contain  olivine  in 
addition  to  their  essential  ingredients;  Hornblende-gabbro  — 
plagioclase  -f  pyroxene  (diallage  or  hypersthene)  +  hornblende  ; 
Mica-norite  =  plagioclase  -f  hypersthene  -f-  biotite  ;  Troctolite  = 
plagioclase -f  olivine.  Amongst  accessory  ingredients  in  all 
the  gabbros  are  apatite,  magnetite,  ilmenite,  garnet,  rutile, 
spinelloids,  etc.  The  gabbros  have  a  mottled  or  speckled 
aspect,  the  felspar  being  usually  bluish-white  to  grey,  and 
the  ferromagnesian  mineral  generally  dark  green. 

The x gabbros  are  usually  more  or  less  altered,  the  felspar  being 
frequently  changed  into  saussurite,  and  the  pyroxene  into  smaragdite  and 

D 


50  STRUCTURAL  AND  FIELD  GEOLOGY 

actinolite.  The  pyroxenes  (diallage  and  hypersthene)  almost  invariably 
exhibit  a  characteristic  pearly  or  submetallic  lustre  (=^schillerisafion\ 
due  to  the  development  of  thin  brown  films  along  their  cleavage-cracks. 
The  gabbros  are  widely  distributed,  occurring  as  plutonic  masses,  sills, 
and  dykes. 

Dolerite  is  for  the  most  part  holocrystalline,  and  varies 
in  texture  from  medium-grained  to  coarsely  granular.  The 
chief  constituents  are  plagioclase,  augite,  and  iron-oxides. 
Occasionally,  some  interstitial  glass  may  occur.  Olivine, 
hypersthene,  biotite,  hornblende  or  quartz  may  be  present, 
and  thus  give  rise  to  varieties,  as  Olivine-dolerite,  Hypersthene- 
dolerite,  Mica-dolerite,  etc. 

The  plagioclase  usually  occurs  as  well-formed  crystals  and  microlites, 
which  penetrate  or  are  enclosed  as  endomorphs  in  the  ferromagnesian 
mineral  (augite  or  olivine).  This  is  known  as  ophitic  structure  (see 
Plate  VIII.  4).  Quartz,  when  present  as  an  original  constituent,  is 
usually  devoid  of  crystallographic  form,  but  is  sometimes  intergrown 
with  the  i&sp'&t  —  micropegmatitic  we granophyric  structure. 

The  common  accessory  ingredients  are  magnetite,  ilmenite, 
and  apatite.  Dolerite  is  a  dark  coloured  rock,  the  medium 
grained  types  being  almost  black  when  fresh,  while  the 
coarser  grained  varieties  are  speckled  dark  green  (or  black) 
and  white  (or  pale  pink).  Owing  to  decomposition  of  the 
ferromagnesian  constituents,  however,  many  dolerites  have 
a  dull,  dark  greenish  colour.  Rocks  of  this  altered  type  are 
known  as  Diabase.  In  these  the  plagioclase  is  often  altered 
into  an  aggregate  of  granules  of  epidote,  calcite,  kaolin,  etc., 
while  the  ferromagnesian  minerals  are  usually  largely  replaced 
by  chlorite,  serpentine,  etc.,  and  the  ilmenite  more  or  less 
changed  into  leucoxene  (titanite).  Dolerite  (Diabase)  occurs 
usually  in  bosses,  sills,  and  dykes,  and  is  very  widely 
distributed. 

Basalt. — This  is  a  greyish-black  to  black,  heavy  rock, 
so  compact,  as  a  rule,  that  the  mineral  constituents  cannot 
be  recognised  by  the  naked  eye.  Frequently,  however,  small 
phenocrysts  are  present,  some  basalts  being  markedly  por- 
phyritic  with  such  minerals  as  plagioclase,  augite,  and  olivine. 

Microscopic  examination  shows  that  the  rock  consists  of  an  aggregate 
of  small  crystals  and  crystalline  granules  of  plagioclase,  augite,  and 
usually  olivine  (see  Plate  IV.).  Almost  constant  accessories  are  magnetite 
and  ilmenite.  Interstitial  glass  is  frequently  present,  plentifully  or 


ROCKS  51 

meagrely  as  the  case  may  be.  Amongst  the  accessory  minerals  occa- 
sionally present,  we  may  note  biotite,  hornblende,  and  hypersthene,  each, 
when  prominent,  giving  rise  to  a  variety  of  the  rock,  as  Mica-^  Hornblende-, 
and  Hypersthene-basalt.  Olivine  is  sometimes  wanting  ;  occasionally  it 
occurs  in  rounded  granular  aggregates  which  may  reach  the  size  of  a 
man's  head.  Such  aggregates  are  often  rich  in  pyroxenes  and 
spinelloids. 

Basalt  is  abundantly  met  with  as  an  intrusive  rock  in 
the  form  of  sills  and  dykes,  and  as  an  effusive  rock  or  lava. 
Like  all  lavas,  effusive  basalt  is  often  more  or  less  vesicular 
and  slaggy,  the  amygdaloidal  cavities  being  lined  and  filled 
with  such  minerals  as  zeolites,  quartz,  chalcedony,  calcite,  etc. 

Tachylite  is  a  basalt-glass  which  is  sometimes  smoothly  homogeneous 
and  compact,  and  at  other  times  highly  porous  and  vesicular.  Basalt- 
Pumice  is  either  foam- like  or  spongiform,  or  may  be  drawn  out  in  the 
form  of  hair-like  threads  containing  long,  cylindrical,  gas  pores. 
Tachylite  sometimes  occurs  as  a  vesicular  crust  on  certain  basalt-lavas, 
the  basal  portions  of  which  are  also  often  highly  vitreous.  The  same 
dark  glass  not  infrequently  forms  the  external  surface  of  basalt-dykes — 
this  "chilled  edge"  varying  in  thickness  from  a  few  lines  to  several 
inches. 

The  diorites  seem  to  be  the  plutonic  equivalents  of  the 
effusive  hornblende-,  mica-,  and  quartz-andesites  (dacites). 
These  andesites,  however,  also  occur  intrusively.  The 
effusive  augite-andesites,  and  the  basalts,  on  the  other  hand, 
being  more  basic,  are  closely  related  to  the  intrusive  dolerites 
and  gabbros.  It  will  be  remembered,  however,  that  basalt 
often  occurs  intrusively,  and  the  same  is  the  case  with  augite- 
andesite. 


3.  ROCKS  WITH  FELSPATHOIDS  TAKING  THE 
PLACE  OF  FELSPARS 

The  only  rocks  belonging  to  this  group  that  need  be 
mentioned  are  Nepheline-basalt  and  Leucite-basalt,  both  of 
which  are  effusive  rocks  of  relatively  recent  geological  age, 
and  having  a  rather  limited  distribution. 

Nepheline-basalt  is  black,  and  composed  essentially  of  nepheline, 
augite,  and  olivine,  with  magnetite,  apatite,  biotite,  and  hatiyne  as 
common  accessories.  Glassy  base  is  bccasionally  present.  Some 
varieties  are  as  compact  and  fine-grained  as  typical  plagioclase-basalt, 
from  which  in  hand-specimens  they  can  hardly  be  distinguished  ;  others 


52  STRUCTURAL  AND  FIELD  GEOLOGY 

have  a  doleritic  aspect.  Phenocrysts  of  olivine  and  augite  are  often 
more  or  less  conspicuous. 

Leucite-basalt,  dark  grey  to  black,  has  for  its  essential  constituents 
leucite,  augite,  and  olivine.  Amongst  the  accessory  ingredients  are 
nepheline,  biotite,  hornblende,  haiiyne,  apatite,  magnetite,  etc.  The  rock 
is  fine-grained,  and  usually  shows  phenocrysts  of  augite  or  olivine  or  both. 
Little  or  no  glassy  base  is  present. 

There  are  several  other  rocks  included  in  this  division,  amongst 
which  are  Nephelinite  =  augite  +  nepheline  ;  Leucitite  =  augite  +  leucite. 

4.  ROCKS  WITHOUT  FELSPARS  OR  FELSPATHOIDS 

These  are  dark  coloured,  heavy,  ultra-basic  rocks — the 
silica  percentage  hardly  averaging  more  than  43,  while  the 
specific  gravity  ranges  between  2-7  and  3-5. 

Limburgite  (Magma-basalt}  is  a  reddish -brown  or  dark  grey  to  black 
rock,  composed  essentially  of  augite  and  olivine  set  in  a  glassy  base. 
The  rock  is  either  fine-grained  or  compact,  with  a  pitch-like  lustre. 
Under  the  microscope  it  is  seen  to  consist  largely  of  glass,  which, 
however,  is  sometimes  so  crowded  with  microlites  of  augite,  magnetite, 
etc.,  that  little  or  no  glass  may  be  visible.  Phenocrysts  of  olivine  and 
augite  are  usually  conspicuous.  Amongst  the  accessory  ingredients 
are  magnetite,  ilmenite,  biotite,  hornblende,  haiiyne,  etc.  Augitite  is 
a  black  rock  composed  essentially  of  augite  and  magnetite  in  a  glassy 
base,  and  resembles  limburgite,  from  which,  however,  it  is  distinguished 
by  the  absence  of  olivine.  Neither  of  these  rocks  is  important  so  far  as 
its  distribution  is  concerned.  The  former  takes  its  name  from  Limburg, 
near  the  Kaiserstuhl  in  Baden,  and  is  met  with  in  various  other  places  in 
Germany.  It  occurs  also  in  southern  Sweden,  Spain,  the  Canary 
Islands,  and  in  Central  Scotland.  Augitite  is  found  in  Bohemia,  Central 
France,  the  Canary  Islands,  the  Cape  Verd  Islands,  and  Ireland. 

Peridot! tes. — These  are  the  most  basic  of  igneous  rocks, 
and  are  composed  mainly  of  olivine — hence  often  designated 
"  divine-rocks."  With  this  constituent  are  associated  small 
proportions  of  one  or  more  of  the  following  minerals — 
picotite,  chromite,  augite,  diallage,  hornblende,  biotite, 
enstatite,  apatite,  magnetite,  ilmenite,  garnet,  etc. 

A  number  of  varieties  have  been  described,  of  which  the  following  may 
be  mentioned  : — Dunite^  composed  almost  wholly  of  olivine  with  a  little 
picotite  or  chromite — occurs  in  the  Dun  Mountain,  S.  New  Zealand  ; 
Picrite^  a  rock  rich  in  idiomorphic  olivine,  with  which  are  associated  in 
varying  amount  pyroxene  (augite,  hypersthene,  enstatite),  hornblende, 
biotite,  magnetite,  ilmenite,  apatite,  and  not  infrequently  a  little  plagio- 
clase.  According  to  the  relative  predominance  of  augite,  hornblende, 


I  XVI. 


i.  SCORIA  OR  CINDER  (GRAND  CANARY).     About  half  natural  si; 


2.   SCORIA  OR  CINDER  (TENERIFFE).     About  natural  size. 


['la  lace  page  52. 


ROCKS  53 

enstatite,  or  biotite,  we  have  augite-,  hornblende-,  enstatite-,  or  mica- 
picrite ;  the  picrites  are  sparingly  represented  in  the  British  Islands ; 
Lherzolite  (from  L'herz  in  the  Pyrenees)  =  oli  vine  +  enstatite  -flight  green 
pyroxene,  with  some  accessory  spinelloid  and  a  little  magnetite  and  apatite. 

The  magma-basalts  are  volcanic  rocks,  mostly  of  late 
Tertiary  age,  occurring,  like  ordinary  basalt,  both  effusively 
and  intrusively.  The  peridotites  are  usually  intrusive,  and 
closely  related  to  the  dolerites  and  gabbros,  into  which  they 
graduate  by  the  increase  of  felspathic  constituents.  Bosses  of 
gabbro  and  dolerite  are  not  infrequently  bordered  by  olivine- 
rock,  into  which  they  pass — the  two  kinds  of  rock  obviously 
being  different  phases  of  one  and  the  same  intrusive  mass. 

The  olivine-rocks  are  often  highly  serpentinised — the 
olivine  being  more  or  less  readily  altered.  Thus  many 
massive  serpentines  are  merely  highly  altered  igneous  rocks 
(see  p.  23). 

B.  Fragmental  Igneous  (Pyroclastic)  Rocks 

These  rocks  include  the  various  kinds  of  material  which 
have  been  ejected  from  volcanoes  in  the  form  of  blocks, 
scoriae,  lapilli,  bombs,  sand,  and  ash.  Blocks  and  Lapilli  are 
the  names  given  to  the  larger  and  smaller  rock-fragments, 
which  may  be  angular  or  subangular ;  while  the  finer  grained 
materials  are  known  as  Sand  and  Ash.  Scoriae  are  loose 
pieces  of  cindery  lava  (Plate  XVI.).  Bombs  are  elliptical 
or  pear-shaped  fragments,  often  vesicular  or  hollow  (see 
Plates  XVII.,  XVII I.).  They  are  simply  clots  torn  from 
the  surface  of  a  mass  of  molten  rock  by  the  explosive  energy 
of  steam,  and  ejected  from  the  crater  of  a  volcano,  their 
form  being  doubtless  the  result  of  their  rotatory  motion. 
Accumulations  of  these  and  other  kinds  of  volcanic  ejecta 
frequently  become  indurated,  and  are  met  with  in  regular  and 
irregular  beds  associated  with  crystalline  igneous  rocks  of  all 
geological  periods.  Volcanic  Agglomerate  is  the  name  given 
to  a  coarse  admixture  of  large  and  small  blocks  and  stones 
set  in  a  matrix  of  comminuted  rock  debris  and  grit,  which 
may  be  either  abundant  or  meagre.  Frequently,  this  rock  is 
found  occupying  the  pipes  or  throats  of  ancient  volcanoes — 
the  upper  portions  of  which  have  been  denuded  away. 


54  STRUCTURAL  AND  FIELD  GEOLOGY 

Volcanic  Breccia  is  a  mass  composed  of  angular  fragments 
of  volcanic  rock.  Volcanic  Tuff  is  the  name  given  to 
aggregates  of  the  finer  grained  ejectamenta  (Plates  XV. ; 
XX.  i).  These  are  often  arranged  in  layers  and  beds  which 
have  been  spread  out  by  water  action.  There  are  endless 
varieties  of  structure  and  texture — some  tuffs  consisting  of 
lapilli,  or  of  grit,  or  of  sand  and  ash ;  others  made  up  of  all 
four,  and  not  infrequently  arranged  in  lenticular  layers  and 
beds.  Some  volcanic  tuffs  consist  of  the  finest  ash-like 
material,  forming  a  dull,  fine-grained  or  compact  rock,  which 
varies  in  colour  from  white  or  grey,  to  darker  or  lighter 
shades  of  red,  blue,  yellow,  etc. 

As  tuffs  are  composed  almost  exclusively  of  fragments  and 
the  comminuted  debris  of  lava,  they  naturally  differ  in  char- 
acter according  to  that  of  the  rocks  from  which  they  have 
been  derived.  Hence  we  have  basalt-tuff,  andesite-tuff, 
rkyolite-tuff,  etc.,  any  of  which  may  of  course  contain  a  large 
or  small  proportion  of  fragments  and  debris  of  sedimentary 
rocks.  Frequently,  however,  the  latter  are  entirely  absent. 


CHAPTER   IV 
ROCKS—  continued 

Classification  of  Derivative  Rocks  : — I.  Mechanically  formed  Rocks,  in- 
cluding Subaerial  and  ^Eolian,  Sedimentary,  and  Glacial  Rocks  (Soil 
and  Subsoil,  Rock-rubble,  Rain-wash,  etc.,  Blown  Sand  and  Dust, 
Laterite,  Terra  Rossa,  Conglomerate,  Grit  and  Sandstone,  Greywacke, 
Clay,  Till,  etc.).  II.  Chemically  formed  Rocks — (Stalactites  and 
Stalagmites,  Tufa,  Magnesian  Limestone,  Rock-salt,  Gypsum, 
Siliceous  Sinter,  Flint,  etc.,  Ironstones).  III.  Organically  derived 
Rocks — (Limestone,  Coal,  etc.,  Guano,  Coprolites). 

II.  DERIVATIVE  ROCKS 

THE  rocks  included  under  this  head  are  of  very  diverse  origin, 
and  show  every  variety  of  composition,  texture,  and  structure. 
Some  are  dominantly  siliceous,  calcareous,  argillaceous,  ferru- 
ginous, or  carbonaceous ;  others  are  mixtures  of  many  differ- 
ent kinds  of  material ;  while  a  few  are  composed  of  one 
mineral  substance  only.  As  regards  texture,  they  vary  from 
smoothly  compact  rocks  to  aggregates  of  the  coarsest  kind. 
So,  likewise,  they  exhibit  much  variety  of  structure — the  large 
majority  consisting  of  fragmental  (clastic)  materials,  while  not 
a  few  are  crystalline  or  subcrystalline.  All  derivative  rocks 
are  of  epigene  origin,  i.e.  they  have  been  produced  at  or  near 
the  surface  of  the  earth  by  the  action  of  the  various  super- 
ficial agents  of  change — wind,  rain,  frost,  water,  etc.  Hence 
many  have  been  formed  mechanically ;  others,  again,  are  due 
to  chemical  action ;  while  yet  others  are  of  organic  origin. 
As  a  rule  they  are  characterised  by  a  more  or  less  pronounced 
bedded  arrangement,  and  hence  are  often  termed  collectively 
the  "  Stratified  Rocks."  Furthermore,  as  water  has  played 
the  most  important  part  in  their  formation,  they  are  not 
infrequently  spoken  of  as  the  "  Aqueous  Rocks."  As  some, 

55 


56  STRUCTURAL  AND  FIELD  GEOLOGY 

however,  show  no  trace  of  aqueous  action,  while  certain 
others  are  not  stratified  or  arranged  in  layers,  we  may 
designate  the  class  by  the  more  comprehensive  term  of 
Derivative  Rocks.  For,  as  we  shall  learn,  all  the  rocks  in 
question  are  composed  of  materials  derived  from  the  breaking- 
up  and  disintegration  of  pre-existing  minerals  and  rocks 
by  epigene  agents,  and  from  the  debris  of  plants  and  animals. 
Various  systems  of  classification  have  been  adopted  for 
the  Derivative  Rocks,  none  of  which  can  be  said  to  be  quite 
satisfactory.  Perhaps  as  convenient  a  system  as  any  is  that 
which  is  based  on  the  geological  origin  of  the  various  rocks. 
At  all  events,  it  has  the  merit  of  directing  the  student's 
attention  to  the  action  of  the  various  epigene  agents  of 
change  which  are  so  ceaselessly  employed  in  modifying  the 
crust  of  the  globe.  We  shall  therefore  group  the  series 
under  these  three  heads  : — MECHANICALLY  FORMED,  CHEMI- 
CALLY FORMED,  and  ORGANICALLY  DERIVED,  Rocks. 

I.  Mechanically  formed  Rocks 

The  vast  majority  of  these  rocks  consist  of  fragmental 
materials — they  are,  in  short,  aggregates  of  fragments  of 
minerals  and  rocks.  Some  of  them  are  due  to  "  weathering  " 
and  the  action  of  wind ;  others  are  the  products  of  the  action 
of  moving  water ;  while  yet  others  are  the  result  of  the 
action  of  ice.  We  have  thus  three  types  of  mechanically 
formed  derivative  rocks,  namely,  i.  Subaerial  and  ^olian 
Rocks,  2.  Sedimentary  Rocks,  and  3.  Glacial  Rocks. 

i.  SUBAERIAL  AND  ^OLIAN  ROCKS 

Under  this  head  are  included  all  accumulations  which  are 
due  to  "  weathering "  and  the  action  of  wind.  The  process 
known  as  "  weathering  "  is  by  no  means,  however,  exclusively 
mechanical.  The  subaerial  disintegration  of  rocks  is  brought 
about  by  the  operation  of  various  agents,  and  it  is  not  always 
possible  to  assign  to  each  its  proper  share  in  the  work 
performed.  In  cold  regions,  rocks  are  broken  down  chiefly  by 
the  action  of  frost ;  in  many  temperate  countries  the  chemical 
action  of  rain  is  often  the  most  effective  agent  of  destruction, 
or  the  work  of  demolition  may  be  pretty  equally  divided 


VOLCANIC  BOMBS.    CINDER  BUTTES,  IDAHO. 

From  Bull.  U.S.  Geol.  Survey,  No.  199. 


Xvm. 


SECTION  OF  VOLCANIC  BOMB.     Nearly  natural  size. 


[Between  pages  56  and  57. 


ROCKS  57 

between  rain  and  frost ;  while  in  hot  and  dry  deserts,  rocks 
crumble  away  mainly  under  the  influence  of  insolation — they 
expand  when  heated  up  during  the  day,  and  contract  more 
or  less  rapidly  at  night,  and  thus  their  constituent  ingredients 
lose  cohesion  and  the  rocks  become  disintegrated.  Some 
or  all  of  these  operations,  it  is  obvious,  may  be  carried 
on  concurrently,  almost  anywhere,  so  that  it  is  usual  to 
include  them  under  the  general  term  of  weathering.  Of 
subaerial  and  aeolian  rocks,  the  most  important  are  the 
following  :— 

Soil  and  Subsoil. — The  particular  origin  of  these  will 
be  fully  discussed  in  Chapter  XXIV.,  and  they  need  only  be 
shortly  defined  at  present.  Subsoil  is  an  unconsolidated  hetero- 
geneous aggregate  of  disintegrated  rock-material ;  while  Soil 
is  essentially  the  same,  with  the  addition  of  organic  matter. 

Rock-rubble  is  the  general  term  applied  to  collections  of 
angular  fragments  which  owe  their  origin  chiefly  to  the  action 
of  frost  in  high  northern  and  temperate  regions,  and  mainly 
to  insolation  in  low  latitudes.  Familiar  examples  are  the 
taluses  of  stony  debris  which  gather  at  the  base  of  precipice 
and  scaur,  and  the  sheets  of  angular  rock-rubbish  which 
curtain  the  hill-slopes  of  our  mountain  areas.  A  rubble  of 
angular  fragments  formed  in  this  way,  if  cemented  together, 
would  be  called  Scree-breccia.  [There  are  many  kinds  of 
breccia — this  term  being  qualified  in  each  case  by  some 
adjective  descriptive  of  its  origin  or  the  character  of  its 
dominant  components.] 

Rain-wash,  Brick-earth,  etc. — In  this  and  other  temper- 
ate regions  the  finer  grained  material  derived  from  the 
disintegration  of  rocks  by  weathering  is  gradually  washed 
down  by  rain  from  higher  to  lower  levels,  and  tends  to 
accumulate  on  gentle  slopes  and  in  hollows.  This  rain-wash 
is  occasionally  sufficiently  fine-grained  and  plastic  to  serve 
for  brick-making  purposes  (bvick-eartK).  But  rain-wash  may 
on  the  other  hand,  consist  of  very  coarse  materials.  In  some 
countries  where  the  rainfall  is  crowded  into  a  short  space  of 
time,  sudden  torrential  rains  sweep  the  steeper  declivities  of 
the  land  bare  of  rock-debris,  and  spread  the  materials  over 
the  low-lying  tracts  that  extend  outwards  from  the  hills  and 
mountains.  The  stones  included  in  such  accumulations  are 


58  STRUCTURAL  AND  FIELD  GEOLOGY 

more  or  less  angular,  having  travelled  usually  no  great 
distance. 

Besides  rain-wash  and  brick-earth  there  are  various  other 
products  of  the  weathering  of  rocks,  but  these  will  be  con- 
sidered later  on  when  we  come  to  discuss  the  nature  and 
origin  of  soils  and  subsoils. 

Laterite  is  a  red  or  brown,  porous  or  cellular,  ferruginous 
clay,  common  in  India  and  other  humid  tropical  countries. 
The  ferruginous  constituent  may  be  diffused  equally  through 
the  mass,  or  aggregated  irregularly.  Laterite  is  readily  dug 
up,  but  becomes  very  hard  when  dried.  It  is  the  product  of 
the  subaerial  decomposition  of  various  rocks,  such  as  gneiss, 
mica-schist,  and  other  crystalline  schists,  diorite,  basalt,  and 
other  eruptive  rocks. 

Terra  rossa  is  a  red  or  brownish  ferruginous  earth  met 
with  more  or  less  abundantly  in  regions  composed  of  limestone 
and  calcareous  rocks.  It  is  simply  the  insoluble  residue 
derived  from  the  dissolution  of  these  rocks  by  atmospheric 
action.  It  assumes  a  great  development  in  the  limestone 
regions  of  southern  Europe,  but  may  occur  wherever  such 
rocks  are  exposed  to  the  action  of  the  weather.  The  red 
earth,  so  frequently  met  with  in  limestone  caves,  is  of  the  same 
origin,  and  has  been  introduced  for  the  most  part  by  rain  and 
melting  snow,  through  fissures  communicating  with  the 
surface. 

Blown  Sand  and  Dust.— Blown  Sand  accumulates  under 
all  conditions  of  climate — wherever,  indeed,  loose  sand  is 
exposed  to  deflation  or  the  transporting  action  of  the  wind. 
Hence  dunes  and  sheets  of  wind-blown  sand  are  well  developed 
upon  certain  sea-coasts  and  lake-shores,  and  in  the  broad,  flat 
valleys  of  many  large  rivers.  In  such  regions,  however,  the 
wind  acts  chiefly  as  a  transporter  of  disintegrated  rock- 
material — the  sand  having  already  been  prepared  for  it  by 
the  action  of  other  superficial  agents,  such  as  tidal  currents, 
waves,  rivers,  etc.  In  dry,  desert  tracts,  however,  blown  sands 
owe  their  origin  and  distribution  mainly  to  the  combined 
action  of  insolation  and  deflation.  By  alternate  expansion 
and  contraction  rock-surfaces  are  broken  up  and  comminuted, 
and  the  grit  and  sand  thus  formed  are  carried  forward  by  the 
wind.  This  loose  material,  swept  against  upstanding  rocks, 


ROCKS  59 

acts  as  a  kind  of  sand-blast  which  abrades,  frets,  honeycombs, 
and  undermines  them — in  other  words,  the  sand  that  results 
from  the  action  of  insolation  grinds  and  reduces  to  sand  and 
dust  the  exposed  rock-surfaces  against  which  it  is  borne.  As 
the  travelling  sand-grains,  which  seldom  rise  more  than  a 
few  feet  above  the  surface,  are  continually  subject  to  mutual 
attrition,  both  in  the  air  and  upon  the  ground,  they  tend  to 
become  more  or  less  well  rounded.  This  character  often 
serves  to  distinguish  desert  blown  sand  from  sand  of  alluvial 
origin — the  smaller  grains  of  which  are  rather  angular  or 
subangular  in  shape.  Having  been  carried  mainly  in  suspen- 
sion, they  escape  the  constant  trituration  to  which  the  grains 
of  blown  sand  are  subject.  Blown  sand,  as  a  rule,  consists 
principally  of  quartz — the  commonest  and  one  of  the  hardest 
of  rock-forming  minerals.  In  coastal  tracts,  however,  the 
dunes,  while  consisting  chiefly  of  quartz,  often  contain  many 
other  ingredients,  more  especially  comminuted  shell-debris. 
Further,  the  grains  of  coastal  blown  sand  are  not  infrequently 
coarser  and  less  well  rounded  than  those  of  desert  sand. 

Dust  is  pre-eminently  a  product  of  relatively  dry  regions 
and  of  deserts — wherever,  indeed,  the  land  is  naked  or  only 
partially  clothed  with  vegetation,  dust  is  formed,  and  may  be 
swept  up  and  transported  by  the  wind.  While  the  blown 
sand  of  a  desert  rises  only  a  few  feet  or  yards  above  the 
ground,  the  powdery  dust  is  often  swept  upwards  to  a  great 
height,  and  may  be  transported  for  hundreds  or  thousands  of 
miles  from  the  place  of  its  origin.  The  fine-grained,  homo- 
geneous, calcareous,  and  sandy  loam  known  as  Loess,  which 
occupies  'wide  areas  in  middle  and  south-eastern  Europe,  and 
covers  vast  tracts  in  China,  is  supposed  by  many  geologists 
to  be  essentially  a  dust  deposit  or  "  steppe  formation." 

2.  SEDIMENTARY  ROCKS 

The  rocks  included  under  this  head  owe  their  origin  to  the 
mechanical  action  of  water,  and  are  usually,  therefore, 
arranged  in  layers  or  beds.  They  vary  exceedingly  in  texture 
— from  coarse  aggregates  of  boulders  and  shingle  to  sedi- 
ments composed  of  the  finest  impalpable  materials.  Less 
sharply  distinguished  from  each  other,  as  a  rule,  than  is  the 


60  STRUCTURAL  AND  FIELD  GEOLOGY 

case  with  igneous  rocks,  sedimentary  rocks  of  various  kinds 
often  merge  into  one  another — coarse-grained  grits  and  sand- 
stones, for  example,  passing  gradually  into  the  finest  argil- 
laceous accumulations.  The  coarser-grained  accumulations 
are  almost  invariably  of  shallow  water  origin — deposited  at 
or  opposite  the  mouths  of  rivers  and  along  the  sea-shore 
between  low-  and  high-water  levels.  The  medium  grained 
masses  have  been  laid  down  generally  in  somewhat  deeper 
water — or  in  places  where  aqueous  action  was  less  strenuous. 
The  finest  grained  sediments  have  accumulated  in  still  water, 
and  therefore  usually  at  some  distance  from  the  land.  Such 
being  the  origin  of  sedimentary  rocks,  it  is  not  surprising  that 
they  should  frequently  contain  the  relics  of  animals  and 
plants,  i.e.  fossil  organic  remains. 

Conglomerate  is  a  bedded  or  amorphous  aggregate  of 
waterworn  stones,  and  may  be  either  of  marine  or  freshwater 
origin.  It  is,  in  short,  simply  a  more  or  less  consolidated 
gravel.  The  matrix  in  which  the  stones  are  set  is  usually 
gritty  or  sandy,  and  may  be  scanty  or  abundant.  The  rock 
often  graduates  into  pebbly  grit  and  conglomeratic  sandstone. 
The  cementing  material  may  be  siliceous,  calcareous,  argil- 
laceous, or  ferruginous.  Quartz  and  hard  siliceous  rocks  are 
usually  the  most  conspicuous  components  of  conglomerate. 
Aqueous  Breccia  is  a  consolidated  rock-rubble,  which  has  been 
accumulated  in  water. 

Grit  and  Sandstone. — These  are  simply  coarser  and 
finer  grained  varieties  of  one  and  the  same  kind  of  rock — 
namely,  compacted  or  cemented  grit  or  sand.  The  most 
abundant  component  is  usually  quartz,  but  many  other 
ingredients  may  be  present.  Amongst  these  may  be  felspar 
(more  or  less  kaolinised),  and  occasionally  some  of  the  less 
readily  decomposed  minerals  derived  from  the  disintegration 
of  igneous  and  schistose  rocks,  such  as  zircon,  schorl,  garnet, 
etc.  The  finer  grains  of  an  aqueous  sandstone,  unlike  those 
of  desert  sand,  are  often  angular  or  subangular,  while  the 
larger  grains  and  small  pebbles  are  usually  well  waterworn 
and  rounded.  Sandstones  may  be  white,  grey,  yellow,  brown, 
red,  greenish,  or  black.  The  colouring  matter  is  in  most 
cases  due  to  that  of  the  cementing  or  binding  material,  which 
may  either  be  dispersed  between  the  grains,  as  in  carbonaceous 


ROCKS  61 

sandstones,  or  appear  as  thin  pellicles  or  skins  enveloping 
the  individual  particles.  White  and  light  grey  sandstones 
and  grits  usually  have  a  calcareous  or  a  siliceous  binding 
material,  or  they  may  have  been  compacted  by  pressure 
alone.  Yellowish  and  brownish  colours  are  due,  for  the 
most  part,  to  ferric  hydrate,  and  red  colours  to  haematite 
(ferric  oxide),  while  greenish  hues  frequently  indicate  the 
presence  of  some  impure  hydrous  silicate  of  iron  and  other 
bases. 

Varieties  : — Freestone  or  Liver-rock^  a  fine-grained  homogeneous  sand- 
stone capable  of  being  tooled  equally  well  in  any  direction.  Flagstone, 
a  thin-bedded,  fine-grained  sandstone,  which  separates  readily  along  the 
bedding-planes,  frequently  more  or  less  argillaceous,  and  often  micaceous. 
Micaceous  sandstone^  fissile,  usually  fine-grained,  with  abundant  scales  of 
white  mica.  Greenland,  a  sandstone  of  a  dull  greenish  colour,  owing  to 
the  presence  of  disseminated  glauconite.  Grit,  simply  a  coarse-grained 
sandstone.  Arkose^  a  sandstone  composed  of  quartz,  felspar,  and  mica  ; 
derived  directly  from  the  disintegration  of  granite  or  gneiss. 

Greywacke  is  a  more  or  less  indurated  rock,  composed  of 
rounded,  subangular,  and  often  sharply  angular  grains  of 
quartz,  felspar,  hornstone,  slate,  and  other  minerals  and  rocks, 
amongst  which  scales  of  mica  are  not  infrequently  conspicuous. 
Besides  such  grains,  greywacke  usually  contains  in  less  or 
greater  abundance  flakes  and  splinters  of  various  compact 
rocks,  such  as  slate,  hornfels,  lydian-stone,  felsite,  etc.  The 
cementing  material  is  usually  meagre  and  often  siliceous,  but 
it  may  be  argillaceous,  calcareous,  or  ferruginous,  or  even 
anthracitic.  Grey  and  blue  are  the  commonest  colours  of  the 
rock,  but  green,  brown,  red,  purple,  yellow,  and  even  black 
varieties  occur.  The  texture  varies  from  compact  and  fine- 
grained to  coarse-grained  and  brecciiform.  The  rock  occurs 
in  thin  layers  and  massive  beds,  interstratified  with  slaty 
shales  and  slates,  and  is  practically  confined  to  the  older 
geological  systems  (Palaeozoic). 

Clays — These  are  aggregates  of  very  finely  divided 
mineral  matter,  which  become  plastic  when  moistened.  The 
finer  varieties  appear  to  the  unassisted  eye  quite  homo- 
geneous, and  when  squeezed  between  the  fingers  have  an 
unctuous  feel,  and  seem  as  if  composed  of  some  impalpable 
substance.  With  the  exception  of  kaolin,  however,  clays  arc 


62  STRUCTURAL  AND  FIELD  GEOLOGY 

really   heterogeneous    aggregates   of    various    minerals    and 
different  kinds  of  rock  material. 

Two  distinct  varieties  of  common  clay  are  recognised — namely, 
alluvial  clay  and  glacio-aqueous  clay.  In  alluvial  clay  the  finer  grained 
constituents  are  obviously  the  result  of  the  chemical  decomposition  of 
minerals  and  rocks,  and  consist  largely  of  hydrous  silicate  of  alumina. 
Disseminated  through  this  material  occur  minute  grains  of  quartz,  and 
frequently  fine  flakes  of  pale  or  colourless  hydrous  mica.  Other  con- 
stituents present  in  ever-varying  proportions  are  iron-oxides  and 
carbonates  of  calcium,  magnesium,  and  potassium.  Clays  of  this 
character  are  invariably  of  secondary  origin  ;  they  have  been  derived  from 
the  disintegration  and  decomposition  of  rocks,  partly  a  mechanical  but 
largely  a  chemical  process.  They  consist,  in  short,  chiefly  of  the  insoluble 
residue  of  rocks.  Glacio-aqueous  day,  on  the  other  hand,  owes  its 
origin  mainly  to  the  mechanical  grinding  and  pulverising  of  rocks  by 
glacier-ice,  and  consists  therefore  chiefly  of  fine  rock-flour  or  rock-meal, 
reasserted  and  deposited  in  water  without  having  undergone  much 
chemical  alteration.  When  clays  of  this  character  are  microscopically 
examined,  we  find  not  only  abundant  grains  of  quartz  and  flakes  of 
relatively  fresh  mica,  but  particles  of  many  other  minerals,  such  as 
various  felspars  and  other  silicates,  all  more  or  less  fresh  and  chemically 
unaltered.  The  proportion  of  hydrous  aluminium-silicate  present  is 
much  less  than  in  the  case  of  clays  of  alluvial  origin.  It  is  generally 
assumed  that  the  hydrous  aluminium-silicate  in  clays  of  all  kinds  is 
kaolin,  but  this  has  not  yet  been  demonstrated.  The  common  belief 
that  the  plasticity  of  clays  is  due  to  the  presence  of  the  hydrous 
silicate  in  question  is  even  more  doubtful.  Almost  any  rock-forming 
silicate — nay,  quartz  itself,  if  reduced  by  grinding  and  rubbing  to  the 
consistency  of  an  impalpable  powder — becomes  plastic  when  moistened, 
and  has  the  earthy  odour  of  clay. 

Varieties  of  Clay. — Kaolin  is  a  product  of  the  decomposition  of  highly 
felspathic  rocks  (granite,  gneiss,  etc.).  The  purer  kinds  of  kaolin  occur 
in  situ — i.e.  they  occupy  the  site  of  the  altered  rock,  and  probably  owe 
their  origin  to  the  action  of  heated  solutions  and  vapours  coming  from 
below.  When  kaolin  has  been  washed  away  from  its  place  of  origin  and 
deposited  elsewhere  it  is  never  so  pure  as  that  which  has  not  travelled, 
but  is  often  mixed  with  many  other  ingredients.  The  purest  kaolin  is  a 
silvery  white  powder,  consisting  entirely  of  very  minute  six-sided  plates 
of  the  mineral  kaolinite — a  hydrous  silicate  of  alumina  with  a  definite 
chemical  formula  (H4Al2Si2O9).  When  moistened,  this  aggregate  of  scaly 
kaolin,  or  Kaolinite -,  is  plastic.  The  Kaolin  or  China-clay  of  commerce, 
however,  usually  contains  many  impurities  ;  it  is  rather  a  rock  than  a 
mineral.  Pipeclay  is  a  fine  white  clay  which  shrinks  too  much  on  the 
application  of  heat  to  be  available  for  pottery-making.  It  contains  a 
larger  percentage  of  siliceous  matter  than  kaolin.  Fire-clay  is  a  clay 
containing  little  or  no  lime,  alkaline  earths,  or  iron,  which  act  as  fluxes. 
It  is  thus  infusible  or  highly  refractory  and  suitable  for  bricks,  etc.,  which 


ROCKS  63 

are  required  to  stand  intense  heat.  Brick-clay  is  an  intimate  admixture 
of  clay  and  sand  with  some  iron-oxide,  and  is  used  for  ordinary  bricks. 
Fuller's  earth  is  a  soft,  dirty  greenish,  brownish,  blue,  yellow,  or  grey 
variety  of  clay,  somewhat  greasy  or  unctuous  to  the  feel,  which  falls  into 
powder  in  water.  Shale  is  the  name  given  to  any  argillaceous  rock  that 
divides  into  thin  layers  or  laminae,  corresponding  to  planes  of  deposition. 
Shales  vary  greatly  in  composition,  some  containing  much  sand  (arena- 
ceous shale\  others  being  largely  carbonaceous  (carbonaceous  shale\  or 
saturated  with  bituminous  matter  (oil  shale}.  Alum  shale  is  an  argil- 
laceous rock  charged  with  a  considerable  quantity  of  disseminated 
pyrite  or  marcasite  (sulphides  of  iron),  through  the  decomposition  of 
which  alum  (sulphate  of  alumina)  and  copperas  or  iron  vitriol  (hydrous 
sulphate  of  iron)  are  formed. 

Loam  is  a  mixture  of  sand  and  clay,  usually  containing 
some  calcium-carbonate,  the  sand  being  plentiful  enough  to 
allow  the  percolation  of  water  through  the  mass.  Most  loams 
are  of  alluvial  origin,  and  are,  therefore,  commonly  developed 
in  valley-bottoms. 

3.  GLACIAL  ROCKS 

These  rocks  are  the  result  of  the  mechanical  action  of  ice. 
Rock-rubble,  already  described  as  a  subaerial  formation,  might 
perhaps  be  classed  as  a  glacial  rock,  since  it  owes  its  origin 
chiefly  to  the  action  of  frost.  It  is  preferable,  however,  to 
include  here  only  those  formations  which  are  the  products 
of  glacial  erosion  and  transport.  Amongst  these,  by  far  the 
most  important  is  Boulder-clay  or  Till,  a  more  or  less 
tenaceous,  gritty  clay,  crowded  with  angular  and  subangular 
stones  and  boulders.  It  varies,  however,  very  much  in 
character,  being  occasionally  more  aranaceous  than  argil- 
laceous. '  When  the  larger  stones  are  removed  it  is  often 
used  for  brick-making ;  in  many  places,  however,  it  is  too 
stony  for  such  a  purpose. 

When  subjected  to  mechanical  analysis,  the  plastic  materials  of  the  till 
of  the  Scottish  lowlands  is  seen  to  be  a  heterogeneous  aggregate  of 
minutely  triturated  mineral  matter,  and  much  rock-flour  of  a  very  fine 
consistency.  Only  a  meagre  proportion  of  this  so-called  "clay"  consists 
of  hydrous  silicate  of  alumina,  or  pure  clay.  Boulder-clay,  in  short,  is 
composed  for  the  most  part  of  unweathered  rock-material — it  is  the  result 
of  glacial  grinding,  and  has  not,  like  ordinary  alluvial  clay,  been  formed 
by  the  chemical  decomposition  of  minerals  and  rocks. 

The  only  other  glacial  accumulation  that  need  be  referred  to  are  the 
mounds  and  sheets  of  earthy  rock-debris  and  boulders  which  occur  in  and 


64  STRUCTURAL  AND  FIELD  GEOLOGY 

opposite  the  mouths  of  many  of  our  mountain  valleys.  These  have  been 
transported  by  the  glaciers  of  the  Ice  Age  as  superficial  moraines,  some 
of  the  more  conspicuous  heaps  having  been  dumped  down  as  terminal 
moraines  during  long  pauses  in  the  final  retreat  of  the  old  ice-flows,  while 
hummocky  sheets  of  the  same  materials  were  gradually  spread  over  the 
flanks  and  bottoms  of  our  mountain  valleys  as  the  glaciers  melted  more  or 
less  rapidly  away. 

II.  Chemically-formed  Rocks 

These  are  for  the  most  part  chemical  precipitates  from 
aqueous  solutions,  and  are  chiefly  calcareous,  siliceous,  ferru- 
ginous, and  saline.  Some  have  been  deposited  at  the  surface 
as  the  result  of  evaporation ;  others  are  precipitates  from 
saturated  solutions,  and  have  thus  accumulated  on  the  floors 
of  salt-lakes  and  seas.  Again,  a  few  are  of  the  nature  of 
aggregations  :  originally  diffused  through  the  rocks  in  which 
they  occur,  they  have  since  drawn  together  and  become  con- 
centrated so  as  to  form  nodules  and  nodular  masses  or 
independent  layers. 

Stalactites,  and  Stalagmites. — These  are  precipitates  from 
water  holding  calcium  carbonate  in  solution.  They  are  of 
common  occurrence  in  limestone  caverns,  the  stalactites 
growing  downwards  from  the  roof  and  the  stalagmites 
gradually  accreting  on  the  floor.  Carbonated  water  percolat- 
ing through  the  limestone  oozes  out  on  the  roof  of  a  cavern, 
and,  being  there  exposed  to  evaporation,  is  compelled  to  part 
with  some  of  its  calcium  carbonate,  which  adheres  to  the 
rock-surface.  When  the  gathering  drop  of  water  falls  to  the 
ground  it  is  there  again  exposed  to  evaporation,  and  gives  up 
the  remainder  of  the  carbonate  which  it  held  in  solution. 
The  colour  of  these  deposits  varies  indefinitely — they  may  be 
creamy-white,  yellowish,  brownish,  or  reddish,  and  are  often 
mottled.  They  usually  show  a  concentric,  laminated  structure, 
and  the  stalactites,  in  the  early  period  of  growth,  are  porous 
and  readily  crushed ;  subsequently,  however,  their  pores 
become  filled  up  with  calcium  carbonate,  and  the  structure 
thus  gradually  solidifies.  Stalagmites  are  seldom  or  never 
so  porous,  but  exhibit  a  well-defined  laminated  structure  (see 
Plate  XIX.).  In  course  of  time  both  stalactites  and  stalag- 
mites, owing  to  molecular  changes,  tend  to  acquire  a  crystalline 
structure. 


(To  face  page  64. 


I.  VOLCANIC  TUFF.     Two-thirds  natural  size. 


2.  SHELLY  LIMESTONE.    Nearly  natural  size. 


[To  face  page  6i-' 


ROCKS  65 

Tufa  or  Calc-sinter  is  formed  by  deposition  from 
calcareous  springs.  It  is  a  porous  and  frequently  very 
friable  compound  of  calcium  carbonate.  The  colour  varies ; 
creamy-white  and  yellow  tints  are  common,  but  red  and 
brown  are  not  infrequent,  while  some  kinds  are  greenish  or 
bluish.  The  rock  is  often  mottled  or  marked  with  concentric 
bands  of  different  colours.  Travertine  is  the  name  given  to 
hard  and  compact  varieties  used  for  building-stones.  They 
have  frequently  a  crystalline  or  subcrystalline  structure. 
Some  tufas  and  travertines  consist  largely  of  small  spherules 
of  calcium  carbonate,  composed  of  concentric  layers  which 
have  been  deposited  successively  around  some  nucleus,  such 
as  a  particle  of  sand  or  a  minute  fragment  of  calcareous  matter. 
When  the  spherules  are  small,  resembling  fish-roe,  the  rock 
is  termed  Oolite  ;  when  they  are  of  the  size  of  peas  the  rock 
is  known  as  Pisolite. 

Dolomite  or  Magnesian  Limestone  is  a  crystalline 
granular  or  earthy  aggregate  of  the  mineral  dolomite  or 
bitter-spar  (double  calcium  and  magnesium  carbonate).  It 
effervesces  only  slightly  with  cold  dilute  hydrochloric  acid, 
but  is  readily  attacked  when  the  acid  is  heated.  Ferrous 
carbonate  and  various  impurities  are  more  or  less  commonly 
present.  When  such  is  the  case  the  rock  is  usually  yellowish ; 
when  impurities  are  only  sparingly  present  it  is  grey  or  white. 
It  frequently  assumes  a  concretionary  structure,  showing 
botryoidal  and  irregular  shaped  masses,  or  appearing  as  if 
built  up  of  spherical  bodies  that  may  vary  in  size  from  small 
marbles  up  to  large  cannon-balls.  Lines  of  bedding  pass 
through  these  curious  concretions.  Many  irregular  shaped 
cavities  appear  in  dolomite,  and  these  are  often  lined  with 
crystals  of  bitter-spar.  Typical  dolomite  is  easily  distin- 
guished from  ordinary  limestone  by  its  superior  hardness 
(3-5  to  4-5),  its  greater  specific  gravity  (2-8  to  2-9),  and  its 
much  less  ready  solubility  in  cold  acid. 

Some  dolomites,  especially  those  which  are  associated  with  beds  and 
layers  of  rock-salt  and  gypsum,  may  be  of  the  nature  of  chemical  precipi- 
tates on  the  floors  of  salt-lakes,  lagoons,  and  other  bodies  of  highly  saline 
water.  Many,  however,  would  appear  to  have  been  originally  common 
limestones  which,  either  at  the  time  of  their  formation  or  subsequently, 
have  by  various  chemical  processes  been  converted  into  magnesian 

E 


66  STRUCTURAL  AND  FIELD  GEOLOGY 

limestones.  Few  limestones  are  without  some  proportion  of  magnesia,  so 
that  it  is  not  possible  to  draw  a  hard-and-fast  line  between  common 
limestone  and  magnesian  limestone.  It  is  only  when  the  magnesia  forms 
20  per  cent,  or  so  of  the  rock  that  the  latter  is  included  among  the 
dolomites  or  magnesian  limestones — but  between  this  and  limestones 
which  contain  little  or  no  magnesia,  there  are  all  gradations. 

Rock-salt  is  a  crystalline,  fibrous,  or  even  granular 
aggregate  of  sodium  chloride,  which  occurs  either  in  thin 
layers  or  in  massive  beds,  sometimes  reaching  a  thickness  of 
several  hundred  feet.  When  pure  it  is  clear  and  colourless, 
but  is  often  stained  with  impurities,  being  frequently  red, 
yellow,  or  grey.  Blue  and  green  tints  also  occur,  but  they 
are  not  common.  The  mineral  is  frequently  turbid  owing 
to  admixture  with  sandy  or  argillaceous  matter.  In  many 
places,  indeed,  it  passes  into  a  saliferous  clay.  It  is  usually 
associated  with  gypsum,  anhydrite,  and  dolomite,  interstrati- 
fied  with  clay,  marl,  and  red  or  particoloured  sandstones,  and 
has  obviously  been  deposited  in  salt  lakes  or  in  arms  of  the 
sea  more  or  less  cut  off  from  the  general  body  of  open  water. 

Gypsum,  hydrous  calcium-sulphate,  occurs  in  beds,  layers, 
or  lenticular  sheets  and  masses,  and  is  often  associated  with 
rock-salt,  anhydrite  (calcium  sulphate),  dolomite,  red  clay, 
and  sandstone,  etc.  In  structure  it  varies  from  compact  to 
granular,  or  it  may  be  a  fibrous,  scaly,  or  sparry  aggregate. 
When  relatively  pure  it  is  white  or  quite  colourless,  but  is 
often  stained  yellow  or  red  by  iron-oxides,  or  coloured  grey 
and  brown,  owing  to  admixture  with  clay  or  other  impurities. 
The  exceedingly  fine-grained  varieties  are  known  as  Alabaster. 
Compact  gypsum  is  readily  distinguished  from  limestone, 
which  it  sometimes  resembles,  for  it  is  scratched  by  the  finger 
nail,  while  limestone  is  not. 

Siliceous  Sinter  is  an  aggregate  of  amorphous  silica  con- 
taining a  variable  proportion  of  water.  It  may  be  loose, 
unconsolidated,  and  porous,  or  dense  and  compact,  and  often 
assumes  stalactitic  and  stalagmitic  forms.  When  free  from 
impurities  it  is  white,  but  as  these  are  often  present  it  may  be 
stained  various  shades  of  yellow  or  red.  It  is  formed  by 
deposition  from  thermal  springs,  as  the  result  of  evaporation ; 
in  some  cases,  however,  deposition  is  partly  due  to  the  action 
of  minute  algae,  which  occasionally  flourish  in  the  hot  pools 
of  a  geyser  region. 


ROCKS  67 

Flint  is  a  hard  grey  or  black  rock,  composed  of  amorphous 
or  chalcedonic  quartz — the  dark  colour  being  due  to  carbon- 
aceous matter.  It  breaks  with  a  marked  conchoidal  fracture, 
and  is  translucent  along  the  sharp  cutting  edges.  Its  most 
characteristic  occurrence  is  in  the  form  of  nodules,  layers,  and 
vertical  ramifying  or  vein-like  masses  in  white  chalk. 

Its  precise  mode  of  formation  is  not  quite  clear,  but  it  would  appear  to 
be  partly  of  organic,  partly  of  chemical,  origin.  Sponges  and  other 
organisms  secrete  soluble  silica  from  sea-water,  and  when  they  die 
additional  silica  is  deposited  upon  and  within  their  skeletons  and 
exuvias.  Calcareous  shells,  and  even  the  chalk  itself  in  which  these  are 
embedded,  have  often  been  partially  or  wholly  replaced  by  silica,  so  that 
silica  in  a  soluble  form  must  have  been  diffused  to  some  extent  through 
the  calcareous  ooze  of  Cretaceous  seas.  Probably  the  silica  was  largely 
derived  from  the  skeletal  remains  of  sponges,  which  flourished  in  great 
abundance  during  the  formation  of  the  Chalk.  Chert  is  a  somewhat 
impure  kind  of  flint,  of  not  uncommon  occurrence  in  limestones  belonging 
to  the  older  geological  systems  (Palaeozoic),  and,  like  it,  probably  partly 
of  organic,  partly  of  chemical,  origin.  In  some  cases,  however,  it  possibly 
represents  the  deposits  of  thermal  springs.  Hornstone  is  a  somewhat 
similar  rock  ;  it  is  more  brittle  than  flint.  Lydian-stone  is  a  mixture  of 
silica  and  clay,  usually  with  carbonaceous  or  ferruginous  matter.  It  is 
black,  purplish,  red,  or  dark  blue,  very  hard  and  compact,  and  often  much 
cracked  and  rent,  the  small  fissures  being  usually  filled  with  white 
quartz.  It  occurs  in  thin  beds  and  layers  in  the  older  Palaeozoic  systems, 
and  in  some  cases,  at  least,  contains  remains  of  radiolarians  (Radiolarian 
Cherf),  so  that  such  rocks  would  appear  to  represent  the  radiolarian  ooze 
of  ancient  seas. 

Ironstones  are  sometimes  of  chemical,  sometimes  of 
organic,  origin,  or  partly  both.  Occasionally  they  occur  in 
the  form  of  beds  or  layers  interstratified  with  other  derivative 
rocks,  or  of  nodules  and  nodular  masses  embedded  chiefly  in 
argillaceous  deposits.  They  are  frequently  met  with  also 
occupying  fissures  and  irregular  cavities.  Limonite,  when 
approximately  pure,  is  a  compact  fibrous  or  stalactitic 
aggregate,  in  which  form  it  usually  occurs  in  veins  and 
cavities.  When  appearing  as  a  bedded  rock,  it  is  usually 
earthy  and  porous,  and  crowded  with  impurities.  It  forms 
the  hardpan  which  so  frequently  appears  under  marshy 
ground  (Bog  Iron-ore)^  and  often  occurs  as  a  lacustrine 
formation  in  layers  of  small  spherical  bodies  (Oolitic  or  Pea 
Iron-ore],  Plcematite,  already  described  as  a  mineral,  appears 


68  STRUCTURAL  AND  FIELD  GEOLOGY 

as  rock-like  masses  in  beds,  veins,  and  cavities  (especially  in 
limestones).  Not  infrequently  it  is  found  replacing  limestone 
(see  under  ORE-FORMATIONS).  As  a  rock  it  usually  contains 
many  impurities,  such  as  clay,  quartz,  oxide  of  manganese, 
etc.  It  passes  into  ferruginous  clay,  etc.  Spathic  Iron-ore  is 
a  granular  or  compact  aggregate  of  siderite,  occurring  in 
beds  and  as  veins,  especially  among  the  older  geological 
systems.  Clay-ironstone  is  a  variety  of  spathic  iron-ore, 
containing  much  clay.  It  is  brown  to  dark  grey  or  black, 
and  appears  as  thin  beds  and  layers,  or  in  the  form  of  balls 
and  nodules.  It  is  a  very  common  rock  among  the 
argillaceous  strata  of  the  Carboniferous  system.  In  some 
cases  it  appears  to  have  been  deposited  on  the  floors  of 
ancient  lakes,  lagoons,  and  estuaries ;  in  other  cases  it  is  of  a 
concretionary  nature  —  the  ferruginous  matter  originally 
diffused  through  an  argillaceous  bed  having  become  aggre- 
gated around  fossils  or  other  foreign  bodies,  so  as  to  form 
nodules  of  various  size.  Many  of  these  nodules  are  septarian 
(Plate  XXVI  I.).  Blackband-ironstone  is  simply  a  clay-iron- 
stone, containing  a  large  proportion  of  carbonaceous  matter 
(from  10  to  52  per  cent.).  Magnetic  Iron-ore  sometimes 
occurs  in  beds  amongst  fossiliferous  strata,  in  which  case  it  has 
resulted  from  the  alteration  of  limonitic  ore.  Magnetic  iron- 
sands  are  often  met  with  in  regions  where  certain  igneous 
rocks  abound,  from  the  disintegration  of  which  the  magnetite 
has  been  derived.  (For  other  occurrences  of  magnetic  iron- 
ore,  see  under  ORE-FORMATIONS.) 

III.  Organically  derived  Rocks 

The  more  important  rocks  included  in  this  division  are 
largely  composed  of  organic  remains — plant  or  animal  as  the 
case  may  be.  Some,  however,  are  due,  or  partly  due,  rather 
to  the  action  of  living  organisms.  Amongst  the  latter  are 
Flint  and  some  kinds  of  Calcareous  Tufa  and  Ironstone. 
The  origin  of  flint  has  already  been  briefly  considered  (p.  67). 
Bog  iron-ore  not  infrequently  owes  its  origin  to  the  action  of 
the  minute  plants  known  as  diatoms,  which  are  able  to 
separate  iron  from  water,  and  to  deposit  it  about  and  within 
their  substance  as  a  hydrate.  Water-loving  plants  are  also 


ROCKS  69 

largely  concerned  in  the  formation  of  calcareous  tufa,  which 
is  not  always  due  entirely  to  the  mere  evaporation  of  aqueous 
solutions.  The  carbonic  acid  which  enables  the  water  to 
hold  the  calcium-carbonate  in  solution  is  decomposed  by  bog- 
mosses  and  their  allies,  and  a  calcareous  crust  is  thus 
gradually  deposited  upon  the  plants.  Many  thick  masses  of 
tufa  have  in  this  way  resulted  partly  from  chemical,  and 
partly  from  organic  action.  Considerable  accumulations  of 
siliceous  sinter  are  likewise  due  in  large  measure,  as  already 
indicated,  to  the  vital  action  of  minute  algae.  But  of  still 
greater  importance  are  the  rocks  which  owe  their  formation 
to  the  action  of  humus  acids  derived  from  the  decomposition 
of  organic  matter.  These  acids  attack  the  iron-bearing 
mineral  constituents  of  rocks,  and  form  ferruginous  solutions  ; 
and  when  such  solutions  are  exposed  to  the  air,  they  are 
oxidised,  and  hydrate  of  iron  (bog  iron-ore)  is  precipitated. 
There  can  also  be  little  doubt  that  organic  acids,  derived  from 
the  decomposition  of  sponges  and  other  forms  of  life,  have 
had  much  to  do  with  the  formation  of  flint  and  chert,  which 
might,  therefore,  be  included  among  organically  derived  rocks 
as  fitly  as  under  chemically  formed  rocks.  The  most  char- 
acteristic representatives  of  the  former  class,  however,  are 
the  calcareous  and  carbonaceous  rocks,  of  which  there  are 
many  varieties. 

Limestone. — Of  this  rock  there  are  innumerable  kinds. 
All  are  composed  essentially  of  carbonate  of  lime,  but  few  do 
not  also  contain  carbonate  of  magnesia.  While  some  are 
very  pure,  others  are  crowded  with  impurities.  They  vary 
greatly  also  as  regards  texture — ranging  from  extremely 
fine-grained  and  compact  rocks  to  coarse  aggregates  of  shells 
and  corals.  Grey  and  greyish-blue  are  the  commonest 
colours,  but  there  are  many  others,  such  as  green,  purple, 
yellow,  red,  grey,  black,  and  pure  white.  While  limestones 
differ  as  regards  their  hardness  and  specific  gravity,  the 
common  and  most  characteristic  types  have  usually  a 
hardness  of  3  or  thereabout,  and  a  specific  gravity  of  2-6  to 
2-8.  One  of  the  best  known  limestones  is  common  Chalk — a 
white  fine-grained  earthy  rock,  generally  soft  and  meagre  to 
the  touch,  soiling  the  fingers.  It  is  largely  composed  of  the 
shells  of  foraminifera,  together  with  the  debris  of  various 


70  STRUCTURAL  AND  FIELD  GEOLOGY 

forms  of  marine  life — more  or  less  reduced  to  the  condition 
of  a  fine  meal  or  flour.  Oolite  is  similarly  composed  of 
organic  debris,  but  is  characterised  by  the  oolitic  structure 
already  described  as  occurring  in  certain  .chemically  formed 
calcareous  deposits.  In  thin  sections  seen  under  the  micro- 
scope the  spherules  show  a  concentric  and  radiated  structure 
— the  latter,  however,  being  sometimes  wanting.  Similar 
spherules  have  been  observed  forming  not  only  in  mineral 
springs  such  as  those  of  Carlsbad  (Sprndelsteiri),  but  in 
shallow  water  in  the  Great  Salt  Lake  of  Utah,  and  on  the 
coral  beaches  of  the  Bahamas.  They  obviously  owe  their 
origin  to  deposition  of  calcareous  matter  on  particles  of  sand 
which  are  kept  in  motion  so  as  to  become  more  or  less 
equally  encrusted.  Shell-marl  is  an  earthy  aggregate  of 
shells  (most  frequently  of  freshwater  origin),  with  a  larger  or 
smaller  proportion  of  argillaceous  matter.  It  often  passes 
into  Lactistrine  Limestone ',  which  is  usually  a  fine-grained, 
dull  white  or  grey  rock — sometimes  earthy,  sometimes 
compact. 

Limestones,  which  are  composed  conspicuously  of  the 
debris  of  crinoids,  corals,  or  shells,  as  the  more  prominent 
ingredients  of  the  rock,  are  known  respectively  as  Crinoidal, 
Coral,  and  Shelly  limestones  (see  Plate  XX.  2).  Occasion- 
ally, the  organic  structure  of  such  rocks  has  been  obscured  or 
even  entirely  effaced  by  subsequent  molecular  changes — the 
mass  becoming  crystalline.  Shelly  limestones  often  acquire 
special  names,  according  to  the  relative  abundance  of  some 
particular  shell,  as  Nummulite-,  Hippurite-,  Ammonite-, 
Gryphcea-limestone,  etc.  Common  Limestone  is  usually  grey 
or  blue,  and  fine-grained  to  compact.  In  many  common  lime- 
stones the  organic  structure  is  only  revealed  in  thin  sections 
under  the  microscope.  On  the  weathered  faces  of  such  rocks, 
however,  fossil  remains  may  often  be  readily  detected.  There 
are  many  varieties  of  common  limestone  characterised  by 
the  presence  of  certain  impurities  and  admixtures.  Amongst 
these  may  be  mentioned  Cornstone,  a  highly  calcareous  sand- 
stone or  arenaceous  limestone,  sufficiently  rich  in  calcium 
carbonate  to  be  burnt  for  lime,  when  a  better  rock  is  not 
available.  Cement-stone  is  a  dull  argillaceous  and  sometimes 
ferruginous  limestone,  often  occurring  in  thin  beds  and  layers 


ROCKS  71 

in  the  same  way  as  clay-ironstone.  It  is  sometimes  used  for 
making  hydraulic  cement.  Carbonaceous  Limestone  contains 
a  considerable  quantity  of  carbonaceous  matter ;  limestones 
of  this  kind  often  emit  a  fetid  odour  when  rubbed  or  struck 
with  a  hammer.  Rotten-stone  is  a  siliceous  limestone,  from 
which  calcium-carbonate  has  been  removed  in  solution,  so  as 
to  leave  only  the  skeleton  of  the  rock. 

Carbonaceous  Rocks. — Under  this  division  are  grouped 
all  accumulations  of  vegetable  debris  and  the  products  of 
their  destructive  distillation  by  natural  causes.  The  least 
mineralised  accumulations  are  included  under  the  head  of 
Peat  or  Turf.  This  might  be  described  as  a  yellow,  brown, 
or  black  aggregate  of  vegetable  debris,  interwoven,  as  it  were, 
and  more  or  less  compressed  and  decomposed.  Impurities 
are  common,  such  as  various  earthy  admixtures,  ochre, 
limonite,  pyrite,  diatomaceous  remains,  etc.,  and  the  amount 
of  ash  after  combustion  is  very  variable.  Lignite  or  Brown 
Coal  is  a  compact  or  earthy  mass,  brown  or  black  with  a 
brown  streak,  and  very  inflammable.  It  differs  from  common 
coal  in  containing  a  greater  proportion  of  bitumen,  or  the 
elements  which  combine  with  carbon  to  form  bitumen.  The 
proportion  of  carbon  in  lignite  ranges  from  55  to  75  per  cent. 
Common  Coal  is  a  black,  compact  carbonaceous  mass,  which 
on  a  fresh  fracture  has  usually  a  resinous  lustre.  It  has  a 
black  streak ;  is  commonly  friable;  is  not  so  inflammable  as 
lignite;  and  contains  75  to  85  per  cent,  of  carbon.  The  per- 
centage of  oxygen,  hydrogen,  and  nitrogen  (the  elements 
which  with  carbon  form  bitumen),  is  lower  than  in  lignite,  but 
higher  than  in  anthracite. 

Varieties  of  Common  Coal. — These  probably  owe  their  distinctive 
characters  to  the  nature  of  the  plants  or  portions  of  plants  of  which  they 
are  composed.  The  chief  kinds  are  Caking-coal,  Cherry-coal,  Splint- 
(Hard-,  or  Steam-)  coal,  and  Cannel-coal.  Caking-coal 'has  a  short,  uneven 
fracture,  while  Cherry -coal  breaks  with  a  clear  shaly  fracture  :  the  former 
fuses  or  runs  together  when  burnt,  the  latter  does  not.  These  are  the 
so-called  "  bituminous  coals."  S flint-coal  has  a  cubical  fracture,  is  not  so 
shattery  or  friable  as  the  bituminous  coals,  and  is  more  difficult  to  ignite 
than  these,  but  has  greater  heating-power.  Cannel-coal  is  smoothly  com- 
pact, breaks  with  a  conchoidal  fracture,  does  not  soil  the  fingers,  burns 
with  a  clear  flame  like  a  candle,  and  crackles  or  chatters  when  burnt — 
hence  the  common  name  Parrot-coal.  Anthracite  (Stone-  or  Blind-coat) 
is  the  most  highly  mineralised  form  of  coal,  consisting  almost  entirely  of 


72  STRUCTURAL  AND  FIELD  GEOLOGY 

carbon.  It  is  black' with  a  vitreous  to  submetallic  lustre  and  a  black 
streak ;  is  not  easily  ignited,  and  burns  almost  without  smoke  or  smell. 
Now  and  again  coal  (of  Carboniferous  age)  has  been  converted  into 
Graphite  (approximately  pure  carbon),  owing  to  the  action  of  intrusive 
igneous  rocks.  In  such  cases  it  has  been  subjected  to  a  kind  of  destruc- 
tive distillation,  all  its  gaseous  elements  having  been  eliminated. 
Graphite  also  occurs  in  lenticular  layers  amongst  crystalline  schists,  but 
its  precise  origin  in  such  conditions  is  uncertain.  Oil-shale  (Bituminous 
schist)  is  a  dark  brown  or  black  highly  bituminous  shale,  which  is  readily 
ignited,  but  cannot  of  itself  be  used  as  a  fuel,  owing  to  its  containing  so 
much  argillaceous  matter.  Asphalt^  an  admixture  of  various  hydrocarbons, 
is  probably  in  most  cases  the  result  of  the  distillation  of  coals  or  other 
organic  matter  by  intrusive  igneous  masses.  It  is  solid  or  highly  viscous 
at  ordinary  temperatures  ;  black  or  brownish-black,  with  a  pitchy  lustre 
and  a  bituminous  odour.  It  occurs  in  sheets  or  layers  interstratified 
with  sedimentary  rocks,  or  as  impregnations  in  such  rocks  as  limestone 
and  sandstone,  or  occupying  fissures  and  cavities  in  the  same.  Not 
infrequently  it  is  met  with  in  veins  traversing  rocks  of  various  kinds  and 
age,  and  in  certain  regions  it  exudes  from  the  ground,  forming  what  are 
known  as  "  tar-springs."  Petroleum  is  a  general  name  for  complex 
hydrocarbons,  which  are  liquid  at  ordinary  temperatures,  but  vary 
greatly  in  this  respect,  some  of  them  being  more  or  less  viscous.  They 
occur  mainly  in  rocks  of  a  porous  character.  The  origin  and  source  of 
the  petroleum  or  rock-oil  derived  from  the  Palaeozoic  rocks  of  North 
America  is  at  present  unknown. 

Guano  is  an  earthy,  white,  grey,  or  yellowish-brown 
accumulation,  with  a  peculiar  odour.  It  consists  mainly  of 
the  excrement  of  birds,  mixed  with  the  offal  and  debris  of 
their  repasts,  and  not  infrequently  with  their  own  remains, 
together  with  those  of  seals,  etc.  It  contains  some  40  to  50 
percent,  of  organic  matter  and  ammonia  salts,  with  19  to  20 
per  cent,  or  thereabout  of  phosphate  of  lime.  Immense 
deposits  of  it  have  been  met  with  in  the  rocky  islets  lying  off 
the  west  coast  of  South  America  (Peru)  and  Africa,  but  these 
are  now  largely  exhausted. 

Leached  Guano. — Guanos  long  exposed  to  the  action  of  rain  tend  to 
have  the  soluble  nitrogenous  constituents  dissolved  out,  so  that  the  rela- 
tively non-soluble  phosphatic  ingredients  become  in  this  way  concen- 
trated. Some  of  the  phosphoric  acid  (in  the  form  of  ammonia-phosphate) 
is  carried  down  into  the  underlying  rock,  which  becomes  phosphatised. 
Limestones,  when  these  are  present,  thus  tend  to  become  converted  into 
tribasic  phosphate  of  lime. 

Rock-guano  is  a  deposit  from  which  all  the  soluble  nitrogenous  pro- 
have  been  leached  out,  so  that  only  calcium-phosphate  remains. 


PLATE  XXI. 


MICROSCOPIC  STRUCTURE  OF  SCHISTOSE  ROCKS. 


*B      -  P 


2. 


1  and  2.  Foliated  Structure  of  Biotite  Gneiss. 

3,  Schistose  Conglomerate.    Nearly  natural  size.    (From  Lehmann's  Entstehung  der  aZtkrystallinischen  i 
Schiefergesteine.) 


Jj  PLATE 


i.  SPOTTED  SLATE.     Nearly  natural  size. 


2.  GNEISS.     About  two-thirds  natural  size. 


[Between  pages  72  and  73. 


ROCKS  73 

This    substance    is    known    in    commerce     as     "rock-guano,"    "rock- 
phosphate,"  etc. 

Coprolites  are  the  droppings  of  fishes,  reptiles,  or 
mammals,  in  a  more  or  less  fossilised  condition.  They  are 
met  with  in  strata  of  widely  different  age,  but  the  coprolites 
of  commerce  are  usually  phosphatic  nodules,  often  of  a  con- 
cretionary nature,  and  not  fossil  excrements.  Phosphatic 
nodules  of  this  kind  sometimes  occur  in  regular  beds,  as  in 
the  Upper  Greensand  and  at  the  base  both  of  the  Red  and 
the  Coralline  Crags  of  England.  They  are  similarly  very 
abundantly  developed  in  the  neighbourhood  of  Charlestown, 
South  Carolina. 

The  origin  of  these  nodules  is  quite  uncertain.  They  often  contain 
fossils,  and  in  some  cases  are  obviously  portions  of  the  rock  underlying 
the  bed  in  which  they  occur.  In  short,  they  are  fragments  of  some 
calcareous  rock,  such  as  Chalk,  which  have  been  rolled  and  rounded,  and 
in  some  way  or  other  have  become  impregnated  with  phosphoric  acid. 
As  this  acid  may  have  been  derived  from  marine  plants,  it  has  been  sug- 
gested that  the  nodules  may  have  become  phosphatised  during  the  long- 
continued  growth  and  decay  of  sea-weeds.  In  many  cases,  however,  the 
nodules  are  certainly  of  a  concretionary  nature,  and  often  contain  no 
fossils.  They  are  frequently  very  hard,  and  may  possibly  represent  the 
residuum  of  ancient  guano-beds — that  is,  the  relatively  insoluble  constitu- 
ents of  such  organic  accumulations,  which  have  been  washed  down  from 
some  neighbouring  land.  Yet  another  source  of  such  concretions  has  been 
suggested.  It  is  known  that  a  small  percentage  of  phosphate  of  lime  of 
organic  origin  is  invariably  present  in  the  marine  organic  oozes  which 
have  been  dredged  from  great  depths,  and  it  has  been  detected  also 
amongst  the  fine-grained  sands  and  muds  which  are  being  accumulated 
around  continental  shores.  It  is  supposed,  therefore,  that  calcareous 
deposits,  such  as  the  phosphatic  chalk  of  this  and  other  countries,  may 
have  derived  their  phosphoric  acid  from  a  similar  source,  and  that  the 
so-called  coprolites  are  simply  concretions  formed  in  the  usual  way  by 
aggregation  of  diffused  mineral  matter. 


CHAPTER  V 
ROCKS — continued 

Metamorphic  Rocks — A.  Schistose  Rocks,  their  General  Characters. 
Quartzose  Rocks.  Argillaceous  Rocks.  Mica-schist.  Gneiss. 
Chlorite-schist.  Talc-schist.  Amphibolites.  Granulite.  Marble. 
Serpentine.  B.  Cataclastic  Rocks,  their  General  Characters. 
Mylonites,  Friction-breccias.  Determination  of  Rocks  in  the  Field. 
General  Characters  of  Argillaceous,  Calcareous,  Siliceous,  and  Fels- 
pathic  Rocks.  Specific  Gravity  of  Rocks. 

III.     METAMORPHIC  ROCKS 
A.  Schistose  Rocks 

THE  rocks  of  this  great  class  are  for  the  most  part 
crystalline,  and  characterised  by  the  structure  known  as 
foliation.  In  a  foliated  rock  or  schist  the  constituent  minerals, 
which  may  consist  of  one  or  of  several  species,  are  arranged 
in  more  or  less  parallel  layers.  Each  layer  may  be  composed 
of  one  kind  of  mineral  only,  or  two  or  more  different  kinds 
may  be  commingled.  The  layers  or  folia  are  usually 
lenticular  in  form — in  some  cases  being  remarkably  even  and 
parallel,  so  as  to  give  the  rock  an  appearance  as  of  fine 
lamination  (see  Plate  XXII.  2).  In  other  cases  the  individual 
folia  thicken- and  thin-out  rapidly  (see  Plate  XXIII.),  alter- 
nate irregularly,  are  often  undulating,  and  frequently  crumpled 
and  puckered.  The  several  folia  are  not  as  a  rule  readily 
separated  from  each  other — the  minerals  of  one  being  usually 
more  or  less  closely  intermingled,  felted,  and  welded  with 
those  of  adjacent  layers  (see  Plate  XXI.  I,  2).  In  all  these 
respects  the  foliation  of  a  schistose  rock  differs  markedly 
from  the  lamination  of  sedimentary  derivative  rocks.  It 
must  also  be  distinguished  from  the  fluxion  or  fluidal 

74 


ROCKS  75 

structure  which  is  so  frequently  present  in  crystalline  igneous 
rocks. 

Metamorphic  rocks  differ  much  as  regards  texture  and 
structure.  They  are  not  all  equally  crystalline  and  foliated. 
Some,  indeed,  show  only  faint  traces  either  of  crystalline  or 
schistose  structure;  others,  again,  although  markedly  crystal- 
line, are  not  foliated.  But  no  foliated  rock  is  devoid  of 
crystalline  texture.  There  can  be  no  doubt  that  many  schists 
originally  existed  as  mechanical  sediments,  and  that  their 
present  constitution  is  the  result  of  subsequent  changes. 
This  is  proved  by  the  simple  fact  that  all  gradations  can  be 
traced  from  unaltered  sedimentary  rocks  into  rocks  which 
become  more  and  more  markedly  crystalline  and  foliated  as 
they  are  followed  in  some  particular  direction.  Again, 
characteristic  clastic  rocks,  such  as  conglomerate,  are  not 
infrequently  met  with  intercalated  between  beds  of  mica- 
schist  or  other  foliated  rock — an  arrangement  obviously 
suggestive  of  the  original  sedimentary  character  of  the  latter. 
Further,  the  occasional  occurrence  of  fossils  in  crystalline 
schists  leaves  no  room  to  doubt  that  such  schists  are  simply 
more  or  less  altered  or  metamorphosed  sedimentary  rocks. 

In  other  cases,  however,  it  can  be  proved  that  certain 
foliated  rocks  are  not  originally  of  sedimentary  origin.  Again 
and  again  we  encounter  massive  igneous  crystalline  rocks, 
such  as  granite,  diorite,  gabbro,  etc.,  passing  outwards  by 
insensible  gradations  into  well-marked  foliated  rocks.  In 
such  cases  we  are  compelled  to  conclude  that  the  foliated 
structure  has  been  superinduced.  In  short,  all  truly  schistose 
masses 'are  metamorphic,  and  originally  existed  either  as 
igneous  or  as  derivative  rocks.* 

Quartzose  Rocks. — Amongst  these  are  included  several 
rocks,  the  clastic  structure  and  sedimentary  character  of 
which  are  more  or  less  conspicuous.  Schistose  Conglomerate^ 
is  an  aggregate  of  waterworn  stones  in  a  crystalline  schistose 

*  It  must  be  noted,  however,  that  a  mass  of  granite  sometimes 
exhibits  along  its  margin  a  kind  of  fluxion  foliation  which  is  original, 
and  not  the  result  of  Tnetamorphism.  See  Chapter  XHi. 

t  The  stones  of  such  conglomerates  are  often  distorted.  They  may 
be  crushed  and  drawn  out  into  mere  lenticles  along  the  planes  of 
foliation,  as  shown  in  Plate  XXI.  3. 


76  STRUCTURAL  AND  FIELD  GEOLOGY 

matrix  (Plate  XXI.  3).  Quartzite  is  composed  of  grains  of 
quartz  cemented  by  silica  to  form  a  very  hard  finely  granular 
or  compact  rock.  It  occasionally  shows  traces  of  false- 
bedding  and  diagonal  lamination,  and  is  obviously  an  altered 
sandstone.  The  fracture  is  usually  splintery,  but  in  very 
compact  varieties  tends  to  be  conchoidal.  The  rock  may  be 
white,  grey,  yellowish,  or  reddish,  and  even  occasionally  bluish 
or  greenish.  It  occurs  in  thin  and  massive  beds  intercalated 
among  crystalline  schists.*  Quartz-schist  is  a  quartzite,  in 
which  a  foliated  structure  has  been  developed — the  planes  of 
foliation  being  glazed  over  with  scales  of  white  mica. 

Argillaceous  Rocks. — The  best  known  members  of  this 
group  are  the  clay-slates.  Clay-slate  is  a  finely  granular  or 
compact  clay-rock.  It  divides  into  thin  plates  which  some- 
times coincide  with  the  original  planes  of  deposition,  but 
usually  cross  these  at  various  angles.  The  colour  of  the  rock 
may  be  blue,  green,  grey,  purple,  brown,  or  red.  Slate  is  com- 
posed mainly  of  argillaceous  matter  (silicates  and  hydrous 
silicates  of  alumina),  but  many  other  ingredients  may  be 
present,  such  as  quartz,  mica,  felspar,  chloritic  and  carbon- 
aceous matter,  rutile,  iron-oxides,  pyrite,  etc.  Some  of  these 
minerals  are  original  constituents  of  the  clay,  others  have 
been  developed  in  the  rock  subsequent  to  its  formation  as  a 
sediment.  The  scales  of  mica  and  crystals  of  rutile,  for 
example,  have  obviously  been  developed  along  the  superin- 
duced planes  of  cleavage.  In  some  slates  well-marked  con- 
spicuous crystals  (chiastolite,  andalusite),  are  disseminated 
irregularly,  and  are  clearly  of  secondary  origin — the  minute 
fragmental  particles  of  the  slate  having  been  pushed  aside 
during  the  gradual  growth  of  the  crystals. 

There  are  many  varieties  of  clay-slate,  such  as  Roofing-slate,  with 
smooth  cleavage  ;  Pencil-slate,  a  soft  slate  of  pure  composition  used  for 
writing  on  slate  ;  \Vhet-slate  or  Novaculitc,  a  highly  siliceous  slate,  witli 
indistinct  cleavage,  used  for  sharpening  knives  ;  Anthracite-slate  and 
Alum-slate,  carbonaceous  slates  which  often  contain  marcasite  and 
pyrite — the  decomposition  of  which  gives  rise  to  the  formation  of  alum  ; 
Knotted-  or  Spotted-slate  (see  Plate  XXII.  i),  a  slate  containing  little 

*  Now  and  again,  however,  sandstone  and  unconsolidated  sand  of 
relatively  recent  age  have  been  partially  converted  into  quartzite  by  the 
infiltration  of  silica  in  solution. 


ROCKS  77 

spots  or  concretionary  knots  which  in  some  cases  seem  to  be  incipient 
stages  in  the  development  of  such  minerals  as  andalusite  and  cordierite  : 
Andalusite-,  Chiastolite-slates,  slates  marked  by  the  presence  of  these 
minerals  in  less  or  greater  abundance.  Spotted  slates  and  andalusite- 
slates,  etc.,  may  be  little  altered  otherwise,  or  they  may  contain  much 
mica,  and  so  gradually  pass  into  (andalusite-mica-schist.  Phyllite  is 
more  crystalline  than  clay-slate.  It  is  a  schistose  clay-rock,  the  cleavage- 
planes  of  which  are  lustrous  with  white  mica,  and  frequently  finely 
wrinkled.  It  is  a  passage-rock  between  clay-slate  and  mica-schist. 
Many  of  the  hard  rocks  included  under  the  term  Hornfels  are  meta- 
morphosed argillaceous  rocks.  [Some,  however,  are  altered  igneous 
rocks,  while  others  appear  to  have  been  originally  impure  limestones 
or  dolomites  (calc-silicate  hornfels]I\ 

Mica-schist  is  a  crystalline  schistose  aggregate  of  mica 
and  quartz,  varying  in  texture  from  fine-grained  to  coarsely 
crystalline.  Of  the  two  constituent  minerals,  sometimes  the 
one  and  sometimes  the  other  predominates,  or  they  may  be 
present  in  approximately  equal  proportions.  The  quartz 
occurs  as  granules  or  granular  aggregates,  and  extends  in 
lenticular  layers,  thinning-  and  swelling-out  more  or  less 
suddenly :  sometimes  it  assumes  the  form  of  irregular 
nodule-like  bodies,  around  which  the  foliated  mica  bends. 
Accessory  minerals  are  of  common  occurrence,  such  as  garnet, 
specular  iron  (iron-mica),  magnetite,  rutile,  schorl,  etc. 

Gneiss  is  a  schistose  aggregate  of  quartz,  felspar,  and 
mica  (muscovite,  biotite — either  or  both).  The  texture  is 
very  variable.  Sometimes  the  rock  is  fine-grained  and  the 
folia  are  thin  and  even  (Plate  XXII.  2):  in  other  cases  the 
folia  may  be  so  thick,  irregular,  or  indistinct  that  in  hand- 
specimens  the  schistose  nature  of  the  rock  may  not  be 
apparent.  The  proportion  of  the  several  component  minerals 
also  varies  indefinitely — one  or  other  often  greatly  pre- 
dominating. Accessory  minerals  are  common,  such  as 
garnet,  apatite,  iron-ores,  schorl,  rutile,  etc. 

Many  varieties  are  recognised  : — as,  Hornblende -gneiss,  with  horn- 
blende and  often  mica  ;  Augite-gnciss,  with  a  pale  green  pyroxene  and 
little  or  no  mica  ;  Protogine-gneiss,  with  a  hydrous  mica  in  place  of 
mica ;  Graphite-gneiss,  with  graphite  replacing  mica  in  whole  or 
part ;  Chlorite-gneiss,  with  chlorite  instead  of  mica  ;  Granite-gneiss, 
with  indistinct  foliation  ;  Augen-gneiss  (Eye-gneiss)  with  large  eye-like 
kernels  (phacoids)  of  quartz  or  orthoclase  (see  Plate  XXIII.). 

The   gneisses   in   which   a   foliated   structure   is  well  developed  are 


78  STRUCTURAL  AND  FIELD  GEOLOGY 

believed  to  be  true  metamorphic  rocks,  and  they  frequently  are  dovetailed 
with  or  graduate  into  mica-schist,  and  even  into  sedimentary  rocks. 
There  are  certain  coarse-grained  gneisses,  however,  which  are  less 
markedly  schistose,  but  show  a  rudely  parallel  banded  structure  that 
seems  comparable  to  the  banded  structure  seen  in  some  massive 
igneous  rocks.  Gneisses  of  this  character  may  therefore  be  truly  eruptive 
rocks.  [The  term  gneiss  is  frequently  applied  to  any  coarsely  crystalline 
granitoid  schistose  rock.] 

Chlorite-schist  is  a  schistose  aggregate  of  scaly  chlorite, 
usually  with  quartz,  and  often  with  felspar,  talc,  mica,  actinolite, 
or  magnetite — the  last  being  frequently  disseminated  as 
perfect  octahedra. 

Talc-schist  is  a  green  to  greenish-grey  or  yellow  schistose 
rock,  very  soft,  and  with  a  pronounced  unctuous  or  soapy  feel. 
It  consists  chiefly  of  scaly  talc,  with  which  quartz  and 
chlorite,  or  mica  are  often  associated.  Other  minerals  may 
be  present,  such  as  felspar,  actinolite,  magnetite,  magnesite — 
the  last  often  in  large  rhombohedrons.  Potstone  is  the  name 
given  to  a  massive  fine-grained  to  compact  aggregate  of  talc 
and  chlorite. 

Amphibolites  are  either  schistose  or  massive — their  chief 
ingredient  being  hornblende  or  actinolite.  The  texture 
varies  from  fine-grained  or  compact  to  coarsely  crystalline. 
Many  other  minerals  may  accompany  the  amphiboles,  such 
as  felspar,  quartz,  pyroxene,  garnet,  epidote,  mica,  rutile, 
sphene,  magnetite,  etc. 

Hornblende-schist,  a  schistose  aggregate  of  hornblende,  usually 
contains  some  felspar  ;  quartz,  black  mica,  and  other  minerals  may  also 
be  present.  The  rock  is  dark  green  to  black.  Usually  the  folia  are 
thick,  but  sometimes  they  are  very  fine,  and  the  schist  then  assumes 
almost  a  slate-like  character.  When  the  foliation  is  indistinct  or  not 
apparent  we  have  the  variety  known  as  Hornblende-rock.  Actinolitc- 
schist  is  composed  mainly  of  light  or  dark  green  actinolite,  with  which 
felspar  and  other  minerals  may  be  associated.  When  the  schistose 
structure  is  obscure  or  wanting,  we  have  Actinolite-rock.  Some  of  the 
dark  fine-grained  amphibolites  are  hardly  to  be  distinguished  in  hand- 
specimens  from  aphanite. 

Granulite.* — A   finely  schistose   holocrystalline  aggregate  of  felspar 


*  There  is  unfortunately  some  confusion  as  to  the  use  of  the  term 
granulite.  Some  French  geologists  restrict  the  term  to  a  fine-grained 
granite  in  which  the  mica  and  sometimes  the  quartz  are  hardly  recognis- 
able by  the  unassisted  eye — so  that  the  rock  seems  to  consist  almost 


ROCKS  79 

and  quartz,  usually  with  small  red  garnets  more  or  less  abundantly  dis- 
seminated. Other  minerals  are  often  present,  such  as  biotite,  muscovite, 
sillimanite,  kyanite,  or  tourmaline. 

Eclogite.— A  medium  to  coarsely  crystalline  mixture  of  scaly  ompha- 
cite  (pyroxene),  with  red  garnet.  Frequent  accessories  are  kyanite, 
white  mica,  smaragdite  and  other  amphiboles,  quartz,  apatite,  rutile, 
zircon,  sphene,  magnetite,  etc.  Foliation  is  usually  obscure,  and 
frequently  wanting. 

Marble  is  a  crystalline  granular  aggregate  of  calcite,  the 
granules  being  of  approximately  equal  size  (Plate  IX.  2). 
The  rock  may  be  white  or  various  tints  of  red,  blue,  yellow, 
green,  or  black,  and  it  is  often  streaked  or  mottled.  Scales  of 
talc  or  white  mica  are  frequently  present,  and  such  minerals 
as  garnet,  tremolite,  actinolite,  zoisite,  biotite,  etc.,  may  be 
disseminated  through  the  rock.  Marble  is  a  metamorphosed 
limestone,  and  the  minerals  referred  to  represent  the  original 
impurities  of  the  limestone  (clay,  sand,  etc.).  In  some  cases 
very  impure  limestones  have  been  transformed  into  a  fine 
smoothly  compact  variety  of  Hornfels*  which  often  consists 
largely  of  an  aggregate  of  various  silicates,  as  tremolite, 
actinolite,  epidote,  garnet,  etc.,  and  quartz. 

Serpentine  is  a  compact  or  fine-grained  rock  which  is 
readily  scratched.  On  freshly  broken  surfaces  it  has  a  faintly 
glimmering  lustre.  It  is  usually  of  a  dull  greenish  colour, 
but  brown,  red,  and  mottled  varieties  are  not  uncommon. 

entirely  of  felspar — the  mica  and  not  infrequently  the  quartz  being  of 
microscopic  dimensions.  Again  a  granite  with  two  micas  is  sometimes 
called  granulite.  Further,  the  term  granulitic  is  commonly  applied 
to  a  fine-grained  rock,  the  crystalline  granules  of  which  present  the 
appearance  in  polarised  light  of  a  brilliant  mosaic.  This  structure  is 
characteristic  of  metamorphic  rocks  which  have  been  subjected  to 
intense  crushing. 

*  Hornfels  is  the  name  given  by  German  geologists  to  certain  rocks 
which  owe  their  origin  to  thermal  or  contact  metamorphism.  They 
vary  much  in  composition,  texture,  and  structure,  and  get  different 
names  according  to  the  nature  of  the  dominant  or  most  prominent 
constituent  mineral,  as  Andalusite-,  Garnet-,  Tourmaline-,  Calc-silicate- 
Hornfels,  etc.  Many  of  the  rocks  are  very  compact  and  so  extremely 
fine-grained  as  to  have  a  flinty  or  horny  aspect :  others  are  more  or 
less  conspicuously  crystalline.  While  some  are  somewhat  schistose, 
others  are  totally  devoid  of  all  traces  of  foliation.  They  occur  usually 
in  immediate  contact  with  batholiths,  or  intrusive  masses  of  granite, 
syenite,  diabase,  etc. 


80  STRUCTURAL  AND  FIELD  GEOLOGY 

It  consists  mainly  of  the  mineral  serpentine,  and  is  often 
traversed  by  numerous  branching  veinlets  of  chrysotile — a 
fine  fibrous  variety  of  serpentine  (Plate  VI.).  In  many  cases 
serpentine  is  the  result  of  the  alteration  of  peridotites  (olivine- 
rocks)  or  other  basic  igneous  rocks,  traces  of  their  original 
mineral  constituents  (such  as  olivine,  pyroxenes,  amphiboles, 
spinelloids,  felspars,  etc.)  being  often  more  or  less  readily 
distinguished.  In  other  cases  the  original  character  of  the 
rock  is  not  apparent.  Such  serpentines  are  often  foliated, 
and  occur  intefbedded  with  schistose  rocks,  where  they  are 
frequently  associated  with  crystalline  limestone. 

'.';••-       '  •    i      -    .     ; 

B.  Cataclastic  Rocks 

The  origin  of  metamorphic  rocks  will  be  considered  in 
Chapter  XV.  Here  it  is  only  necessary  to  point  out  that 
many  of  these  rocks  have  acquired  their  present  structure  and 
texture  under  the  influence  of  the  intense  strains  and  stresses 
to  which  the  crust  of  the  earth  has  been  subjected.  Under 
enormous  pressure  the  constituent  minerals  of  certain  rocks 
have  been  rendered  in  a  sense  plastic  and  compelled  to  flow. 
Such  rocks  have  consequently  acquired  a  "shear-structure" 
not  unlike  the  fluxion-structure  developed  in  lavas.  Rocks  of 
this  kind  are  schistose,  and  usually  holocrystalline.  In  other 
cases,  however,  a  rock  subjected  to  intense  compression  has 
been  merely  crushed  into  fragments  or  even  pulverised,  with- 
out acquiring  a  thoroughly  crystalline  character.  No  hard 
and  fast  line  separates  these  two  kinds  of  structure.  In  point 
of  fact,  one  often  passes  by  insensible  gradations  into  the 
other.  Typical  examples  of  the  former  are  crystalline  and 
schistose ;  while  similar  examples  of  the  latter  are  fragmental 
or  cataclastic.  But  such  cataclastic  rocks  often  exhibit  a 
"streaky "or  even  schistose  character,  and  their  constituents 
may  to  some  extent  show  a  superinduced  crystalline  texture. 
In  many  cases,  however,  they  consist  of  a  compacted  aggre- 
gate of  smaller  and  larger  angular  and  subangular  fragments, 
forming  finer  or  coarser  brecciiform  masses,  which  are  neither 
crystalline  nor  schistose.  Rocks  of  this  type  are  often  well 
developed  along  one  or  both  sides  of  faults  or  dislocations  of 
the  crust  (see  Chapter  XL). 


xxm: 


[To  /ace 


ROCKS  81 

Mylonites. — This  is  the  name  given  to  more  or  less  fine- 
grained cataclastic  rocks.  They  are  typically  developed  along 
the  lines  of  great  overthrusts  or  reversed  faults,  and  are 
usually  closely  associated  with  crystalline  schistose  rocks,  into 
which  indeed  they  often  pass.  Most  frequently  they  show 
well-developed  "shear-structure" — the  rock  being  composed 
mostly  of  minute  fragments  and  particles  with  now  and  again 
larger  fragments,  set  in  a  streaky  groundmass  of  crushed 
materials.  When  the  nature  of  the  larger  fragments  (of 
minerals  or  rock)  is  obvious,  not  infrequently  one  is  able  to 
say  what  the  original  uncrushed  rock  may  have  been. 

Friction-  or  Crush-breccia. — This  is  an  aggregate  of 
angular  and  subangular  fragments,  varying  in  size  up  to  one 
foot  or  even  more  in  diameter.  Breccias  of  this  kind  occur 
in  the  same  way  as  mylonites,  into  which  they  often  pass. 
All  gradations,  indeed,  may  be  traced  from  coarse  breccia 
into  mylonites,  and  from  the  latter  into  schists.  When  there 
has  been  considerable  movement  of  the  rock-debris  and  the 
fragments  have  been  rolled  over  and  more  or  less  rounded,  the 
rock  is  termed  a  friction-conglomerate.  Although  friction- 
breccias  are  best  developed  in  regions  where  the  rocks  have 
been  subject  to  much  compression — to  folding  and  great 
dislocations  and  displacements — and  where  frequently  meta- 
morphism  is  more  or  less  pronounced,  they  are  nevertheless 
not  confined  to  such  regions.  Faults  traversing  strata  of  all 
kinds  are  not  infrequently  accompanied  by  breccias.  Some- 
times these  are  confined  to  a  line  of  fracture,  filling  up  the 
space  between  the  two  walls  of  a  fault ;  while  in  other  cases  the 
rocks  forming  one  or  both  walls  of  a  fault  have  been  jumbled, 
shattered,  and  brecciated.  The  stones  in  such  fault-breccias, 
as  they  are  termed,  are  not  infrequently  rubbed  smooth  and 
striated  on  one  or  more  sides. 

Determination  of  Rocks  in  the  Field 

The  beginner  who  casts  his  eye  for  the  first  time  over  a  good 
collection  of  rock-specimens  is  apt  to  be  dismayed  by  the  numerous 
species  and  varieties  with  which  he  is  expected  to  become  acquainted. 
The  coarser  grained  rocks,  whether  they  be  derivative,  igneous,  or  meta- 
morphic,  do  not  appear  to  present  much  difficulty.  He  may  feel  hopeful 
that,  with  ordinary  care,  he  will  eventually  learn  to  distinguish  one  from 

F 


82  STRUCTURAL  AND  FIELD  GEOLOGY 

another.  It  is  the  fine-grained  and  compact  stones,  in  which  the  naked 
eye  may  fail  to  recognise  the  nature  of  the  component  ingredients,  that 
chiefly  puzzle  him.  How  are  the  various  compact  igneous  rocks  to  be 
distinguished  from  each  other,  or  indeed  from  other  compact  rocks  of 
any  kind  ?  One  may,  of  course,  with  the  help  of  a  pocket-lens,  be  able 
to  detect  the  character  of  some  fine-grained  rocks,  but  in  the  case  of 
many  others  a  careful  microscopical  examination  of  thin  slices  will  be 
absolutely  necessary  before  one  can  be  sure  of  their  nature.  It  must  be 
admitted,  indeed,  that  we  cannot  know  all  about  rocks,  whether  they  be 
coarse-grained  or  fine-grained,  before  we  have  submitted  them  to  careful 
scrutiny  under  a  microscope.  The  student  who  does  not  desire  to 
specialise  as  a  petrologist,  however,  may  nevertheless  come  to  be  very 
knowing  in  the  matter  of  rock- determination  by  following  a  few  simple 
rules  or  methods. 

In  attempting  to  determine  a  rock  in  the  field,  the  beginner  should 
be  careful,  in  the  first  place,  to  examine  fresh  surfaces.  Rocks  which 
have  been  exposed  to  the  weather  are  always  more  or  less  altered,  the 
weathered  crust  being  thick  or  thin,  according  to  the  nature  of  the  rock, 
and  the  length  of  time  it  has  been  subject  to  the  mechanical  and  chemical 
action  of  the  superficial  agents  of  change.  It  need  hardly  be  said,  there- 
fore, that  the  fresh  rock  may  differ  very  much  from  the  crust  which 
covers  it.  Assuming,  then,  that  a  fresh  surface  has  been  obtained,  the 
student  endeavours  to  ascertain  to  which  of  the  great  classes  the  rock 
belongs — is  it  crystalline  or  fragmental?  If  it  be  coarse-grained,  he 
should  have  no  difficulty  in  answering  this  question.  Should  it  be 
crystalline,  the  crystalline  ingredients  will  either  be  confusedly  aggregated 
or  arranged  in  parallel  lenticular  folia.  In  the  former  case  it  is  probably 
an  igneous  rock*  ;  in  the  latter  case  it  is  a  schist.  If,  on  the  other  hand, 
the  rock  be  composed  of  rounded  waterworn  materials,  it  is  a  conglomerate 
of  some  kind  ;  but  if  the  included  fragments  be  angular,  it  is  a  breccia. 
It  is  obvious,  therefore,  that  in  the  case  of  coarse-grained  rocks  the  first 
step  towards  determining  them  is  simple  enough.  With  finer-grained 
rocks,  however,  it  is  otherwise.  At  first  the  observer  may  have  some 
difficulty  in  discriminating  between  the  mineral  constituents  of  such 
rocks  even  with  the  help  of  a  lens.  It  is  a  good  plan  to  pound  a 
chip  of  the  fresh  rock  with  the  hammer,  and  reduce  it  by  rubbing  to  a 
gritty  powder.  In  the  case  of  crystalline  igneous  rocks  and  many  schists, 
this  process  often  succeeds  in  separating  the  rock-constituents  from  each 
other  more  or  less  completely,  so  that  they  can  be  turned  about  with 
the  point  of  a  knife  in  all  directions,  and  subjected  to  a  more  thorough 

*  It  might  be,  however,  coarsely  crystalline  limestone,  dolomite,  or 
anhydrite.  These  rocks  are  simple — i.e.  they  are  composed  throughout 
of  one  mineral  ingredient,  and  could  readily  be  determined  by  the  simple 
tests  mentioned  on  pp.  28,  29.  A  coarsely  crystalline  igneous  rock,  on 
the  other  hand,  would  usually  be  recognised  by  its  composite  character, 
and  by  the  fact  that  its  constituent  minerals  were  not  apparently  affected 
by  dilute  acid. 


ROCKS  83 

examination  than  is  possible  with  the  minerals  locked  together  in  a 
hand-specimen.  A  magnet  drawn  through  the  powdered  rock  will  take 
up  any  magnetite  that  happens  to  be  present.  There  should  be  no 
difficulty  either  in  differentiating  between  minerals  with  good  cleavage, 
and  those  which  do  not  possess  this  property.  Fragments  of  the  former 
will  be  distinguished  by  their  flat  lustrous  faces,  while  the  latter  will  be 
quite  irregular  in  shape.  Should  the  rock  be  too  fine-grained  to  allow 
of  the  rough  separation  of  its  constituents  for  examination  with  the  lens, 
the  powder  will  yet  yield  much  information  when  studied  with  a  low 
power  under  the  microscope.  A  little  should  be  placed  on  a  glass  slide, 
with  a  drop  or  two  of  water  or  oil  added,  and  another  glass-slip  laid 
atop.  By  gently  rubbing  the  upper  slip  over  the  powder,  the  grains 
can  be  still  further  reduced,  and  the  individual  constituents  more 
thoroughly  isolated.  In  this  way  the  observer  will  often  get  evidence 
sufficient  to  enable  him  to  pronounce  on  the  true  nature  of  the  rock.  All 
the  common  rock-forming  minerals  may  be  detected  by  this  simple 
process  just  as  readily  as  they  can  be  by  the  examination  of  carefully 
prepared  rock-slices. 

For  purposes  of  determination  in  the  field  the  more  commonly  occur- 
ring rocks  (those,  namely,  which  enter  most  largely  into  the  formation  of 
the  earth's  crust)  may  be  grouped  under  these  four  heads  :  I.  Argillaceous 
Rocks,  2.  Calcareous  Rocks,  3.  Siliceous  Rocks,  and  4.  Felspathic  Rocks. 

Argillaceous  and  Calcareous  Rocks  are  readily  recognised.  Their 
hardness  ranges  from  less  than  i  up  to  3-5,  and  they  are  thus  all  readily 
scratched  with  a  knife — many  of  them  even  with  the  finger-nail.  A  very 
soft  rock,  having  a  dull  earthy  aspect,  and  which  when  moistened  is 
plastic,  must  be  a  clay.  Should  the  rock  be  somewhat  harder,  and  occur 
in  thin  irregular  laminae,  which  may  or  may  not  cohere,  it  is  an  argil- 
laceous shale.  If  the  shale,  when  rubbed  down,  be  more  or  less  gritty 
from  the  presence  of  grains  of  quartz,  it  is  an  arenaceous  shale.  Or, 
should  it  be  black,  and  without  gritty  matter,  it  is  most  probably  a 
carbonaceous  or  a  bituminous  shale.  Or,  again,  if  it  be  lighter  in  colour 
and  effervesce  with  cold  dilute  hydrochloric  acid,  it  is  a  calcareous  shale. 
Clay-slate  will  be  readily  recognised  by  its  structure — splitting,  as  it  does, 
into  firm  plates  along  the  superinduced  planes  of  cleavage.  Argillaceous 
rocks,  when  breathed  upon,  emit  a  peculiar  earthy  odour,  and  although 
this  character  is  not  confined  to  such  rocks,  nevertheless  if  a  fresh  rock, 
having  this  pronounced  odour,  be  readily  scratched,  and  present  a  dull 
earthy  aspect,  we  may  feel  tolerably  sure  that  it  is  argillaceous. 

Limestones  are  all  easily  scratched  with  a  knife  (the  hardness  being 
3  or  less),  and  effervesce  briskly  with  cold  dilute  acid.  If  the  rock  be 
relatively  pure,  it  will  weather  with  only  a  thin  (generally  yellowish  or 
brownish)  pellicle  for  a  crust  ;  if  it  contains  many  impurities  (clay,  sand, 
iron-oxides),  the  weathered  crust  will  be  correspondingly  thick.  Dolomitic 
limestone  is  slightly  harder  (3  to  4)  than  common  limestone.  It  effer- 
vesces very  slowly  with  cold  acid,  but  more  briskly  when  the  acid  is 
heated. 

The  Siliceous  Rocks  are  distinguished  especially  by  their  hardness. 


84  STRUCTURAL  AND  FIELD  GEOLOGY 

Some  varieties  are  very  fine-grained  or  compact,  so  that  the  constituents 
are  not  visible  even  with  the  help  of  a  lens.  Rocks  of  this  kind  (such  as 
flint  and  chert}  usually  occur  in  the  form  of  nodules,  irregular  aggregates, 
veins,  or  layers,  especially  in  limestones.  They  are  compact  and  homo- 
geneous, have  usually  a  dull,  horny-like  aspect,  cannot  be  scratched  with 
the  hardest  knife,  and  do  not  effervesce  with  acid.  Lydian-stone  is 
another  close-grained  siliceous  rock,  which  usually  contains  carbonaceous 
matter,  so  that  it  is  dull  grey  or  even  black.  It  is  commonly  associated 
with  greywacke,  clay-slate,  or  schistose  rocks,  and  is  often  traversed  by 
numerous  ramifying  veiniets  of  white  quartz.  These  characters  and  its 
great  hardness  suffice  to  distinguish  it.  All  the  compact  siliceous  rocks 
referred  to  are  differentiated  from  certain  compact  igneous  rocks  (felsites) 
with  which  they  might  possibly  be  confounded,  by  their  infusibility 
before  the  blowpipe. 

Granular  siliceous  rocks  are  typically  represented  by  sandstones  and 
conglomerates — the  determination  of  which  presents  no  difficulty  even  to 
a  beginner.  The  hard,  round,  subangular  or  angular  grains  of  a  sand- 
stone consist  chiefly  of  quartz.  The  student  should  be  able,  with  the 
help  of  his  pocket-lens,  to  detect  the  fragmental  character  of  even  the 
finest-grained  sandstone.  The  bedded  character  of  the  rock  and  the 
general  aspect  of  the  strata  with  which  it  is  associated  should  leave  him 
in  no  doubt  as  to  its  nature.  Sandstones,  of  course,  differ  greatly  as 
regards  their  hardness  and  durability — some  are  much  more  closely 
compacted  than  others.  The  nature  of  the  cementing  material,  as  we 
have  already  learned,  also  varies.  It  is  just  possible  that  a  fine-grained 
oolitic  limestone  might  be  mistaken  for  a  sandstone,  but  the  relative 
softness  of  the  former,  and  the  readiness  with  which  it  effervesces  with 
acid,  at  once  betray  its  character.  It  is  unnecessary  to  add  a  word  as  to 
the  determination  of  conglomerates — everyone  is  familiar  with  the 
appearance  of  such  consolidated  gravels.  The  only  other  eminently 
siliceous  rock  that  calls  for  notice  is  quartzite.  This  is  simply  a  much 
indurated  sandstone — the  grains  of  the  rock  being  cemented  by  silica.  It 
is,  therefore,  exceedingly  hard,  and  breaks  with  a  splintery  or  conchoidal 
fracture,  and  usually  shows  a  somewhat  glassy  lustre*  The  student  may 
note  further  that  in  the  case  of  quartzite  and  many  sandstones  which 
have  silica  for  their  binding  material,  the  component  grains  are  so  firmly 
cemented  together  that  they  do  not  separate  but  break  across,  so  that  the 
face  of  the  fracture  is  smooth  and  often  glistening  or  glassy  ;  whereas,  in 
ordinary  sandstones,  a  fresh  fracture  is  dull  and  has  a  rough  feel,  owing 
to  the  rock  separating  between  the  grains  and  not  through  them. 

The  Felspathic  Rocks  offer  a  wide  range  as  regards  hardness,  texture, 
and  structure.  Some  are  soft  and  more  or  less  earthy  ;  others  are  hard, 
distinctly  crystalline,  fine-grained,  smoothly  compact,  or  glassy  ;  while  yet 
others  are  fragmental.  Again,  the  crystalline  varieties  may  be  schistose, 
or  their  ingredients  may  be  confusedly  aggregated.  Many  of  the  harder 
felspathic  rocks  cannot  be  scratched  with  a  knife,  but  usually  their  hard- 
ness is  less  than  that  of  the  siliceous  rocks.  In  the  fresh  state  none  of 
them  effervesces  with  acids. 


ROCKS  85 

(a)  Highly  Vitreous  Felspathic  Rocks  are  easily  determined  by  their 
lustre.  A  glassy  rock  showing  a  uniform  texture,  either  black  or  some 
dark  colour,  which  breaks  usually  with  a  conchoidal  fracture,  and  is 
translucent  on  thin  edges,  is  most  probably  obsidian.  Pitchstone  may 
generally  be  distinguished  from  obsidian  by  its  pitchy  or  resinous  lustre, 
by  its  fracture,  which  is  more  frequently  splintery  than  conchoidal,  and 
by  its  feebler  translucency.  Its  colour  is  variable — dark  or  light  shades 
of  green,  brown,  red,  yellow,  etc.  Obsidian  and  pitchstone  are  acid 
glasses,  and  are,  therefore,  often  found  passing  or  graduating  into  acid 
hemicrystalline  rocks.  The  basic  glasses  are  associated  with  such  rocks 
as  basalt,  into  which  they  pass.  This  is  often  seen,  for  example,  along 
the  marginal  surfaces  of  dykes  and  sills  where,  owing  to  the  chilling 
influence  of  the  surrounding  rocks,  the  molten  mass  has  cooled  too 
rapidly  to  permit  the  development  of  crystallisation. 

(&)  Compact  and  Fine-grained  Felspathic  Rocks  are  usually  very 
difficult  to  determine  in  the  field  ;  indeed,  it  is  often  quite  impossible  to 
tell  one  from  another  in  hand-specimens.  The  mode  of  their  occurrence, 
the  general  character  of  the  rocks  with  which  they  are  associated,  their 
structural  features,  and  their  mode  of  weathering,  will  often  aid  one  in 
forming  an  opinion  as  to  their  nature.  But  only  a  microscopic  examina- 
tion will  suffice  to  determine  precisely  what  the  rocks  really  are.  The 
following  notes  on  some  of  the  more  commonly  occurring  compact 
felspathic  rocks  may,  however,  be  of  use  to  the  beginner. 

A  fine-grained  or  smoothly  compact  rock,  which  on  fresh  surfaces 
can  barely  be  scratched  with  a  knife  or  not  at  all,  which  is  not  affected 
by  acid,  and  is  fusible  in  thin  splinters,  is  probably  &felsite.  Should  it 
contain  more  or  less  numerous  phenocrysts  of  felspar  and  quartz,  and 
possibly  other  minerals,  such  as  biotite  and  hornblende,  it  is  most  likely 
a  quartz-porphyry.  Rocks  of  this  type  usually  weather  with  a  thin  white 
or  light-coloured  crust.  Their  colour  is  very  variable,  but  generally 
not  dark — white,  grey,  yellow,  brown,  or  red,  being  the  prevailing  tints. 
Quartz-porphyry  might  sometimes  be  confounded  with  rhyolite — a  hemi- 
crystalline rock.  Rhyolite^  however,  is  not  nearly  so  common  or  widely 
distributed -a  rock  as  the  former.  It  often  exhibits  a  finely  cavernous 
character — the  cavities  being  lined  with  quartz  or  some  other  form  of 
silica — a  structure  which  is  not  characteristic  of  quartz-porphyry.  While 
the  groundmass  of  a  rhyolite  not  infrequently  has  the  smoothly  compact 
or  dull  horny-like  aspect  of  that  seen  in  felsites  and  quartz-porphyries,  it 
is  more  commonly  glassy,  enamel-like,  or  porcellaneous,  and  very  often 
highly  perlitic  ;  but  spherulitic  and  fluxion  structures  are  more  especially 
characteristic.  Scattered  through  this  matrix  are  conspicuous  crystals 
of  glassy  felspar,  granules  of  quartz,  and  small  crystals  of  some  dark 
ferromagnesian  mineral  (usually  biotite  or  augite).  The  rock  has  often  a 
peculiar,  harsh,  rough  feel.  All  the  highly  acidic  felspathic  rocks  have, 
on  fresh,  unweathered  surfaces,  a  hardness  of  6-5  or  thereabout ;  they  are 
usually  light-coloured,  and  generally  weather  with  white  or  light-coloured 
crusts,  which  are  relatively  thin,  and  more  or  less  well  defined,  but  not 
readily  detached  from  the  rock. 


86  STRUCTURAL  AND  FIELD  GEOLOGY 

The  less  acidic  felspathic  rocks^  like  the  more  acid  types,  are  often 
fine-grained  and  compact — either  sparingly  porphyritic  or  without  any 
conspicuous  phenocrysts.  The  commonest  varieties  in  this  country  are 
the  andesites  and  porphyrites.  These  are  more  frequently  dark-coloured 
than  the  highly  acid  rocks,  but  are  on  the  whole  lighter  in  colour  than 
the  basic  rocks  of  which  basalt  may  be  taken  as  the  type.  Their 
hardness  varies  on  fresh  surfaces  from  5-5  to  6  ;  but  as  few  of  our 
andesites  are  without  some  alteration,  their  hardness  is  often  less  than  6, 
so  that  they  can  be  scratched  with  a  knife.  The  compact  varieties  are 
generally  bluish  or  greenish,  and  tend  to  weather  with  a  thinnish  light- 
coloured  crust,  which  is  often  more  or  less  ferruginous  (yellow  or  brown). 
But  when  they  contain  a  considerable  percentage  of  ferromagnesian 
minerals,  they  are  darker  coloured,  and  the  crust  is  thicker  and  more 
markedly  rusty  in  aspect.  Varieties  of  this  kind  can  hardly  be  dis- 
tinguished from  similar  fine-grained  basalts,  although  there  is  almost 
always  something  in  the  general  aspect  of  an  andesite  as  seen  in  the 
mass,  such  as  the  character  of  its  jointing  and  its  mode  of  weathering, 
which,  after  some  experience,  the  student  will  come  to  recognise. 

Compact  phonolites  are  generally  light-coloured — white,  grey,  bluish, 
or  yellowish — and  emit  a  bell-like  clink  when  struck  with  the  hammer. 
They  are  often  readily  split  up  into  thin  flags,  and  weather  with  a 
well-defined  white  crust. 

Fine-grained  trachytes  are  commonly  light  or  dark  grey  rocks,  having 
the  harsh  or  rough  feel  already  referred  to  as  more  or  less  characteristic 
of  some  rhyolites — which  they  in  this  and  other'respects  closely  resemble. 
They  are  differentiated  from  rhyolites  chiefly  by  the  absence  of  quartz. 
There  is  nothing  in  the  mode  of  their  weathering,  however,  to  distinguish 
them  from  rhyolite.  Neither  trachytes  nor  phonolites  are  common  rocks 
in  Britain.  Large  crystals  of  sanidine  are  common  in  trachytes. 

Compact  diorite  varies  in  colour  from  greyish-green  to  dark  green 
and  black,  and  is  known  as  aphanite — the  green  colour  being  due  to  its 
hornblende,  and  not  necessarily  to  the  presence  of  such  decomposition- 
products  as  serpentine  and  chlorite.  The  fresh  rock  has  a  hardness  of 
about  6.  Such  a  rock  closely  resembles  an  amphibolite,  from  which, 
however,  it  may  sometimes  be  distinguished  by  its  weathering.  If  the 
rock  be  a  diorite  it  will  weather  with  a  rusty  crust,  the  inner  portion  of 
which  (that,  namely,  which  is  nearest  to  the  relatively  unweathered  rock) 
will  exhibit  effervescence  with  acid— thus  revealing  the  presence  of 
calcium  carbonate — one  of  the  products  of  the  decomposition  of  the 
constituent  felspar. 

Compact  and  fine-grained  basic  rocks  are  widely  distributed  in  this 
country.  They  are  chiefly  basalts^  and  in  fresh  exposures  are  very  dark 
blue  to  black  in  colour.  They  have  a  hardness  of  5  to  6,  but  when  the 
rock  is  weathered  the  hardness  may  be  much  less.  Very  often  a  few 
isolated  grains  or  an  occasional  granular  aggregate  of  green  or  yellowish- 
green  olivine  may  be  seen,  even  in  the  most  smoothly  compact  basalt. 
The  jointing  of  the  rock  is  generally  more  regular  than  that  of  the 
compact  and  fine-grained  acid  igneous  rocks — basalt  being  frequently 


ROCKS  87 

divided  by  prismatic  joints,  and  thus  rendered  columnar  in  structure. 
Basalt  weathers  with  a  thick  rusty  or  yellowish-brown  crust,  which  often 
exfoliates  in  concentric  shells.  Not  infrequently,  owing  to  the  alteration 
of  its  ferromagnesian  constituents  into  serpentine  and  chlorite,  the  rock 
assumes  a  dull,  dirty,  greenish  colour. 

(c)  Coarse-grained  Hemicrystalline  Felspathic  Rocks. — These  rocks 
may  show  little  trace  of  a  base  or  groundmass,  and  will  often  puzzle  the 
beginner,  for  it  is  the  character  of  the  finer  grained  matrix  that  in  many 
cases  differentiates  one  type  of  igneous  rock  from  another.  But  with  care 
and  patience  he  may  hope  to  distinguish  most  of  those  the  essential 
components  of  which  can  be  clearly  seen  with  or  without  the  aid  of  his 
pocket-lens.  Very  often  the  nature  of  the  rock  will  be  suggested  by  the 
aspect  of  its  weathered  crust.  When  this  is  light-coloured,  and  the  rock 
has  a  white,  greyish-white,  or  yellowish-white,  earthy,  chalky,  or  clay-like 
appearance,  he  may  suspect  that  he  is  dealing  with  a  more  or  less  acidic 
rock.  Should  it  be  a  quartz-porphyry  or  a  rhyolite,  it  will  show  either 
granules  or  crystals  of  quartz,  which  may  be  readily  separated  from  the 
decomposed  matrix  in  which  they  are  embedded.  If,  on  the  other  hand, 
no  trace  of  quartz  be  visible,  then  the  rock  will  probably  be  orthoclase- 
porphyry,  trachyte,  phonolite,  or  andesite.  In  the  case  of  andesites,  how- 
ever, it  must  be  remembered  that  the  weathered  crust  is  not  infrequently 
rather  brown  and  rusty.  Coarse-grained  basalts,  like  the  finer  grained 
and  compact  varieties,  are  in  like  manner  often  recognisable  by  their 
conspicuous  dark  rusty  brown  or  yellowish  crusts.  Removing  the 
weathered  crust,  the  character  of  which  may  have  suggested  the  type 
of  rock  he  is  examining,  the  observer  carefully  scrutinises  the  component 
minerals  in  the  usual  way.  If  it  be  a  basalt-rock,  it  ought  to  show  on 
a  fresh  fracture  an  aggregate  of  striated  felspars — numerous  glassy-like 
rods — with  entangled  crystals  and  crystalline  granules  of  dark  or  black 
minerals  (augite  and  magnetite),  and  frequently  scattered  granules  of  a 
greenish  mineral  (olivine).  An  ordinary  coarsely  crystalline  andesite 
will  consist  almost  exclusively  of  laths  or  rods  of  a  similar  felspar,  olivine 
being  absent,  and  dark  ferromagnesian  minerals  either  apparently  wanting, 
or,  if  present,  not  nearly  so  numerous  as  in  a  basalt-rock.  When  such 
minerals  abound,  then,  without  careful  microscopic  examination,  it  would 
be  impossible  to  say  whether  the  rock  was  basaltic  or  andesitic.  The 
trachytes  and  rhyolites,  as  we  have  learned,  are  usually  lighter  coloured 
rocks  than  the  andesites  and  basalts.  Both  are  commonly  distinguished 
by  their  rough  feel.  Their  dominant  felspar  is  orthoclase,  of  which  the 
phanerocrystalline  types  appear  to  the  naked  eye  to  be  almost  entirely 
composed.  Usually,  however,  we  may  detect  small  crystals  of  dark 
ferromagnesian  minerals  more  or  less  sparingly  entangled  among  the 
felspar  crystals.  Very  often,  too,  large  phenocrysts  of  glassy  orthoclase 
(sanidine)  are  present.  Should  quartz  be  also  present,  then  the  rock  is 
a  rhyolite  ;  if  it  be  wanting,  we  have  a  trachyte.  Decomposing phonolite 
can  hardly  be  distinguished  from  weathered  trachyte.  But  the  coarser 
grained  kinds,  on  freshly  fractured  surfaces,  usually  show  well-defined 
tabular  crystals  of  glassy  felspar,  often  arranged  in  parallel  positions,  and 


88  STRUCTURAL  AND  FIELD  GEOLOGY 

having  as  common  associates  six-sided  prisms  of  nepheline,  and  not 
infrequently  prisms  of  some  pale  green  pyroxene  and  small  crystals  of 
magnetite.  But  dark-coloured  minerals  are  not  as  a  rule  so  common 
as  in  trachyte.  The  rock  is  prone  to  become  decomposed,  the  decom- 
position-products, in  the  form  of  various  zeolites,  appearing  in  cracks 
and  cavities.  Quartz-porphyries,  which,  to  the  unassisted  eye,  may  seem 
to  consist  exclusively  of  crystalline  ingredients,  have  a  granitoid  aspect. 
On  fresh  faces  the  observer  will  readily  distinguish  the  two  dominant 
minerals,  orthoclase  and  quartz,  while  on  weathered  faces  the  presence 
of  a  groundmass  is  often  revealed  by  earthy  or  clay-like  matter  entangled 
between  the  quartz  and  the  weathered  felspar. 

(d]  Coarse-grained   Holocrystalline  Felspathic   Rocks    offer    on    the 
whole  fewer  difficulties  to  the  beginner.     If  he  has  already  learned  to 
recognise   the   common   rock-forming   minerals,  he   should  be   able   to 
distinguish  the  several  essential  constituents  of  such  rocks  as  coarse- 
grained granite,  syenite,  diorite,  gabbro,  and  dolerite.     The  finer  grained 
holocrystalline   varieties   will   sometimes    be   diagnosed  with   difficulty. 
In  such  cases  he  will  often  be  aided  by  the  appearances  presented  by  the 
weathered  crusts.     The  felspars,  he  will  remember,  tend  to  be  decomposed 
into  an  earthy  or  clay-like   substance,  which   will   be   lightly  or  more 
darkly  tinted  according  to  the  proportion  of  decomposing  ferromagnesian 
constituents  with  which  they  are  associated. 

(e)  Fragmental  Igneous  Rocks  vary  greatly  as  regards  texture,  some 
being  exceedingly  fine-grained,  while  others  are  composed  of  an  aggregate 
of  larger  and  smaller  blocks.     The  finer  grained  varieties  are  usually  of 
an  earthy  or  clay-like  aspect  and  readily  scratched  ;  many,  indeed,  are  so 
slightly  compacted  that  they  may  be  disintegrated  between  the  fingers. 
Such  rocks  may  show  scattered  through  them  flakes  of  mica,  and  broken 
crystals   of  various   volcanic  minerals.     They  are  usually  well-bedded, 
having  been  arranged  and  spread  out  in  layers  by  aqueous  action.     For 
the  same  reason  they  often  dovetail  with  or  pass  into  ordinary  aqueous 
rocks,  such  as  sandstones  and  shales.     Many  tuffs,  again,  consist  largely 
of  finely  comminuted  debris  of  igneous  rocks,  either  of  one  or  of  different 
kinds  ;  such  tuffs  commonly  contain  cinders  and  lapilli  of  lavas.     Rocks 
of  this  class  are  most  usually  interbedded  with  lava-form  igneous  rocks. 
The   coarser  agglomerates  and   volcanic   breccias  may  also  occupy   a 
bedded   position,   but   they   very   frequently  occur   in   the  pipes  of  old 
volcanoes  or  in  masses  immediately  surrounding  these. 

(/)  Crystalline  Schists. — Their  determination  is,  as  a  rule,  not  hard. 
Where  the  several  ingredients  are  conspicuous,  and  the  rock  tolerably 
fresh,  its  diagnosis  should  not  be  more  difficult  than  that  of  a  coarse- 
grained holocrystalline  igneous  rock.  Not  infrequently,  however,  the 
component  crystals  of  a  schist  are  very  small  individuals,  and  so  closely 
intermingled,  that  they  can  hardly  be  distinguished  even  with  the  help 
of  one's  pocket-lens.  In  such  cases,  should  the  rock  be  a  pale  whitish- 
green  colour,  have  a  marked  soapy  feel,  and  be  easily  scratched,  it  will 
probably  be  talc-schist.  Chlorite-schist  is  also  easily  scratched,  but  is 
dark  green  and  not  quite  so  unctuous  to  the  touch.  Hydro-mica-schist  is 


ROCKS 


89 


likewise  soft,  pale  white  or  greenish,  and  with  only  a  slightly  soapy  feel. 
A  rock  composed  of  a  fine-grained  schistose  aggregate  of  dark  green  or 
greenish-black  fibrous  scales  is  probably  hornblende-schist;  a  similar 
aggregate  of  dark  or  light  green  acicular  or  ray-like  fibrous  crystals  is 
probably  actinolite-schist.  The  other  common  schistose  rocks— mica- 
schist  and  gneiss — should  not  be  hard  to  distinguish,  even  in  fine-grained 
varieties — seeing  that  the  constituent  minerals  are  all  easily  recognisable, 
especially  on  the  edges  of  the  folia. 

The  Argillaceous,  Calcareous,  Siliceous,  and  Felspathic  rocks 
undoubtedly  form  the  bulk  of  the  earth's  crust — the  other  kinds  of  rock 
not  included  under  one  or  other  of  those  heads  taking  quite  a  subordinate 
place.  Many  of  them,  however,  are  of  great  economic  importance — 
conspicuous  members  of  the  series  being  the  coals,  carbonaceous  com- 
pounds of  all  kinds,  ironstones,  and  various  minerals,  which  now  and 
again  play  the  part  of  rocks.  Among  the  latter  are  rock-salt,  gypsum, 
anhydrite,  apatite,  etc.,  but  as  the  distinguishing  characters  of  the 
minerals  have  already  been  given,  they  need  not  be  repeated  here.  If 
the  student  can  recognise  the  minerals  in  small  specimens,  he  should  not 
have  much  difficulty  in  diagnosing  them  when  they  appear  as  massive 
aggregates. 

The  specific  gravity  of  rocks  is  not  infrequently  a  character  of  some 
importance,  and  of  no  little  assistance  in  their  determination.  The 
following  table  gives  the  average  specific  gravity  of  a  number  of 
representative  igneous  rocks  *  : — 


Granite 

2-6—27 

Diorite 

2-7—2-9 

Quartz-porphyry  . 

2-4  —  2-6 

Andesite 

2-4—2-7 

Rhyolite 

2.4—2-5 

Gabbro 

2.7—3-0 

Obsidian 

2-0  2-3 

Dolerite 

2-7—3-0 

Pitchstone    . 

2-3—2-4 

Basalt. 

2-8—3.1 

Syenite 

2-6  —  2*9 

Nepheline-basalt 

2-9—  3-0 

Orthoclase-porphyry    . 

2-6  —  2-7 

Leucite-basalt 

2-8—2-9 

Trachyte 

2-4  —  2-6 

Limburgite  . 

2*8  —  3-0 

Phonolite      . 

2-5  —  2-6 

Peridotites   . 

3-0—3-5 

*  The  student  who  desires  to  take  the  specific  gravity  of  a  rock 
cannot  do  better  than  employ  the  simple  and  satisfactory  instrument 
described  in  Appendix  C. 


CHAPTER  VI 

FOSSILS 

Modes  of  Preservation  of  Organic  Remains.  Kinds  of  Rock  in  which 
Fossils  occur.  Fossils  chiefly  of  Marine  Origin.  Importance  of 
Fossils  in  Geological  Investigations.  Climatic  and  Geographical 
Conditions  and  Terrestrial  Movements  deduced  from  Fossils. 
Geological  Chronology  and  Fossils. 

HITHERTO  we  have  been  concerned  with  rocks  mainly  as 
aggregates  of  mineral  matter,  and  only  a  passing  reference 
has  been  made  to  the  fact  that  certain  derivative  accumula- 
tions contain  fossils — the  remains  and  traces  of  formerly 
living  creatures.  We  have  seen,  it  is  true,  that  some  kinds 
of  rock,  such  as  coal  and  limestone,  consist  chiefly  of  the 
debris  of  plants  and  animals,  but  we  have  now  to  realise  that 
almost  every  variety  of  derivative  rock  may  be  more  or  less 
fossiliferous,  and  that  traces  of  former  life  have  been  met 
with,  now  and  again,  even  in  certain  igneous  and  metamorphic 
rocks. 

When  a  plant  or  animal,  or  any  portion  of  either,  is  buried 
in  sediment,  it  becomes  subject  to  decomposition.  This 
process  usually  results  in  the  destruction  of  all  organic  com- 
pounds of  carbon  and  nitrogen,  and  even  the  harder  and 
more  durable  parts  undergo  some  change,  and  may  eventually 
become  disintegrated,  and  entirely  disappear.  Certain 
chemical  changes,  however,  may  supervene  before  the  process 
of  destruction  is  completed.  In  many  cases,  for  example, 
carbonisation  takes  place — various  gases  are  given  off,  and 
the  organic  tissues  are -gradually  transformed  into  carbon.  Or 
mineral  matter  may  be  introduced  in  solution  so  as  to  fill  up 
all  the  cavities  of  the  original  structures,  or  even  to  replace 
completely  the  substance  of  the  organism.  Fossils,  therefore, 

90 


FOSSILS  91 

are  met  wtth  in  all  states  of  preservation.  Exceptionally,  the 
entire  organism  has  been  preserved  with  little  or  practically 
no  change  of  the  original  substance — the  bodies  having  been 
protected  from  decomposition  by  the  nature  of  the  materials  in 
which  they  have  been  entombed.  As  examples  may  be  cited 
the  carcasses  of  the  extinct  mammoth  and  woolly  rhinoceros 
which,  long  ages  ago,  were  sealed  up  in  the  frozen  earths  and 
ice  of  Northern  Siberia,  so  that  when  in  recent  times  they 
became  exposed,  owing  to  the  gradual  dissolution  of  the 
medium  in  which  they  had  been  buried,  their  bodies  were  in 
so  fresh  a  state  that  dogs  devoured  the  flesh.  Insects,  spiders, 
and  plants,  have  similarly  been  completely  preserved  in 
amber  (fossil  gum  or  resin) ;  but,  in  most  cases,  these  would 
appear  to  be  more  or  less  carbonised.  The  more  common 
methods  of  preservation,  however,  are  as  follows  : — 

Incrustation. — The  organism  under  certain  conditions  is 
enveloped  in  a  covering  of  mineral  matter.  Calcareous  tufa, 
for  example,  is  often  precipitated  upon  plants  growing  near 
springs  containing  much  calcium-carbonate.  In  the  case  of 
thermal  waters  siliceous  sinter  may  be  the  incrusting  substance. 
Vegetable  and  insect  remains  preserved  in  this  manner  are 
often  more  or  less  carbonised,  or  they  may  be  entirely 
decomposed  and  dissipated,  leaving  merely  hollow  moulds 
behind  them. 

Carbonisation. — Plant-remains  and  chitinous  animal  struc- 
tures, without  having  been  previously  incrusted,  frequently 
undergo  carbonisation — a  deoxydising  process  which  takes 
place  under  conditions  permitting  of  only  a  limited  access  of 
air.  Thus  plants  accumulated  in  marshy  ground,  or  on  the 
floor  of  lake  or  estuary,  or  buried  in  mud,  etc.,  tend  to  under- 
go a  kind  of  distillation  whereby  the  oxygen  and  other  gases 
are  gradually  eliminated — the  carbon  in  this  way  becoming 
concentrated. 

Moulds  and  Casts. — The  substance  of  a  buried  organic 
body  may  be  entirely  dissipated,  and  only  a  mould  of  it 
remain.  Should  this  mould  be  subsequently  filled  with 
mineral  matter,  a  cast  showing  the  external  form  of  the 
original  will  be  produced.  This  is  a  common  kind  of  fossilisa- 
tion.  Many  fossil  shells,  for  example,  are  simply  casts,  and 
do  not  contain  a  particle  of  the  original  substance.  When 


92  STRUCTURAL  AND  FIELD  GEOLOGY 

an  empty  bivalve  or  univalve  shell  is  enclosed  in  a  deposit, 
the  sediment  usually  at  the  same  time  fills  the  vacuity. 
Afterwards,  the  shell  itself  may  be  gradually  dissolved  and 
removed  by  percolating  water.  The  cavity  thus  formed  may 
be  subsequently  reoccupied  by  mineral  matter,  and  in  this 
way  a  perfect  cast  will  be  produced.  Not  infrequently,  how- 
ever, the  space  left  by  the  shell  remains  unfilled,  containing 
in  its  centre  the  stony  kernel  which  formerly  occupied  the 
interior  of  the  original.  Should  this  kernel  not  adhere  to 
the  matrix,  it  will  rattle,  like  a  nut  in  its  shell,  when  the 
specimen  containing  the  fossil  is  shaken. 

Permeation  and  Molecular  Replacement.  —  Mineral 
matter  has  often  thoroughly  permeated  an  organic  body, 
and  filled  up  all  its  pores  and  cavities — a  process  which 
has  usually  been  preceded,  accompanied,  or  followed  by 
carbonisation.  Not  infrequently,  under  these  conditions,  the 
original  substance  itself  is  more  or  less  molecularly  replaced 
by  mineral  matter,  with  partial  or  perfect  preservation  of  the 
internal  structure.  This  kind  of  fossilisation  is  well  illustrated 
by  some  specimens  of  silicified  wood,  the  minutest  structures 
of  which  have  been  so  completely  replaced  that  a  slice  of  the 
specimen,  viewed  under  the  microscope,  reveals  as  much  as 
a  section  of  the  original  wood  itself  could  have  shown. 
Permeation  and  molecular  replacement  may  be  exemplified 
by  one  and  the  same  fossil,  so  that  the  two  kinds  of 
fossilisation  are  frequently  hard  to  distinguish.  An  organic 
body  which  is  permeated  and  molecularly  replaced  by  mineral 
matter  is  a  true  petrifaction. 

In  cases  of  true  petrifaction,  the  replacing  mineral  is 
usually  either  silica  (mainly  chalcedony  or  opal)  or  calcium 
carbonate.  The  same  substances  also  play  the  most 
important  part  in  the  formation  of  incrustations  and  casts, 
which  is  just  what  might  have  been  expected  when  we 
remember  how  very  widely  calcareous  and  siliceous  solutions 
are  diffused.  Other  substances,  however,  not  infrequently 
replace  organic  remains,  such  as  the  compounds  of  iron 
(pyrite,  marcasite,  haematite,  limonite,  and  siderite),  and,  less 
frequently,  gypsum,  barytes,  fluor-spar,  and  various  metals 
and  metallic  compounds. 

It  is  not  only  the  relics  and  remains  of  plants  and  animals 


FOSSILS  93 

which  are  termed  fossils,  but  any  recognisable  trace  of  their 
former  existence — any  impressions  or  tokens  left  behind 
them — whether  it  be  footprints,  tracks,  or  trails,  burrowings, 
castings,  or  coprolites,  or  even  the  markings  traced  on  sedi- 
ment by  the  waving  to-and-fro  of  seaweeds,  etc. — are  all 
equally  fossils. 

Kinds  of  Rock  in  which  Fossils  occur. — As  a  rule, 
the  best  preserved  fossils  are  met  with  in  the  finer  grained 
sedimentary  rocks,  as  in  marls,  limestones,  clay  and  shale, 
and  fine  argillaceous  or  calcareous  sandstones. 

Calcareous  Rocks. — Argillaceous  limestones  and  marly 
shales  are  often  highly  fossiliferous,  and  the  fossils  are 
usually  well  preserved.  But  pure  limestones,  which  have 
become  more  or  less  crystalline,  frequently  appear  to  be 
poor  in  organic  remains,  so  that  when  a  fresh  fracture  of 
the  rock  is  obtained,  few  or  no  traces  of  any  structure  may 
be  visible.  On  surfaces  which  have  been  for  some  time 
exposed  to  the  weather,  however,  fossils  not  infrequently 
project  in  bold  relief — the  limey  matrix  in  which  they  are 
embedded  offering  less  resistance  to  atmospheric  action. 
The  same  phenomena  characterise  many  dolomitic  lime- 
stones. 

Argillaceous  Shales. — Not  infrequently  these  are  rich  in 
fossils — their  impervious  character  having  doubtless  tended 
to  the  preservation  of  the  remains.  Some  shales,  however, 
are  very  barren,  or  the  few  fossils  present  may  be  included  in 
nodular  concretions  of  calcium-carbonate,  siderite,  or  other 
substance. 

Sandstones  are  not  so  frequently  fossiliferous  as  shales,  for 
which  there  are  at  least  two  reasons.  First,  a  sandy  sea-floor, 
owing  to  frequent  or  constant  movement  of  the  sediment,  is 
not  favourable  to  sedentary  forms  of  life,  and  is  therefore 
avoided  by  organisms  which  cannot  shift  for  themselves.  An 
ordinary  siliceous  sandstone  might  therefore  be  expected  to 
be  somewhat  barren.  Second,  the  permeable  character  of 
sandstones  must  favour  the  subsequent  passage  of  percolating 
water  which  so  frequently  dissolves  and  removes  organic 
bodies.  Massive  thick-bedded  quartzose  sandstones  and 
red  sandstones  are,  as  a  rule,  singularly  poor  in  organic 
remains.  Certain  thin-bedded  argillaceous  and  thick-bedded 


94  STRUCTURAL  AND  FIELD  GEOLOGY 

calcareous  sandstones,  however,  are  not  infrequently  highly 
fossiliferous — and  this  is  especially  the  case  when  the 
sandstones  occur  in  beds  alternating  and  interosculating 
with  dark  carbonaceous  or  lighter  coloured  calcareous  shales. 

Conglomerates  are  generally  unfossiliferous,  or,  if  fossils 
are  present,  these  are  usually  more  or  less  rolled  and  water- 
worn.  For  example,  we  may  obtain,  in  some  Carboniferous 
and  Jurassic  conglomerates,  worn  fragments  of  the  trunks 
and  branches  of  trees — but  the  more  delicate  twigs  and 
leaves  are  absent.  So,  again,  in  gravels  and  conglomerates 
of  Pleistocene  and  Recent  age,  only  the  more  resistant  large 
bones  and  teeth  of  mammals  are  ever  met  with,  and  they  are 
often  rolled  and  broken.  There  are  exceptions  to  every  rule, 
however,  for,  now  and  again,  tolerably  well-preserved  shells  do 
occur  in  conglomerates. 

Volcanic  Tuffs. — In  certain  bedded  volcanic  tuffs  fossils 
occur,  but  this  is  not  common.  Plant-remains  have  even  been 
encountered  in  the  coarse  tuffs  and  agglomerates  that  occupy 
thejthroats  or  necks  of  certain  ancient  Carboniferous  volcanoes 
in  Scotland.  Probably  these  represent  trees,  etc.,  which  grew 
upon  the  slopes  of  the  old  cones  after  the  volcanoes  had 
become  extinct.  More  rarely  still,  charred  fragments  of  trees 
have  been  met  with  enclosed  in  the  lower  portion  of  an 
ancient  lava. 

Schistose  Rocks.—  It  need  hardly  be  said  that  these  rocks 
are  usually  destitute  of  organic  remains.  Nevertheless,  fossils 
are  occasionally  present  in  schists,  as  in  certain  metamorphic 
Silurian  rocks  in  the  neighbourhood  of  Christiania,  and  in 
the  highly  crystalline  schists  of  Liassic  age  which  enter  into 
the  structure  of  the  Central  Alps. 

Fossils  differ  much  not  only  in  regard  to  the  state  of 
preservation  of  their  internal  structure,  but  also  of  their 
external  form.  In  many  cases,  they  have  been  much  com- 
pressed— what  were  formerly  cylindrical  branches,  for  example, 
have  often  been  flattened,  so  as  to  give  lenticular  sections 
when  cut  across.  In  limestones,  marly  shales,  and  calcareous 
sandstones,  shells,  corals,  etc.,  usually  retain  their  original 
shapes ;  while  in  argillaceous  shales,  fossils  of  all  kinds  are 
apt  to  be  more  or  less  flattened — a  rule,  however,  which  has 
many  exceptions.  In  clay-slates  and  rocks  which  have 


FOSSILS  95 

obviously  been   subjected   to   much   compression,  fossils   are 
usually  highly  distorted  and  often  recognised  with  difficulty. 

By  far  the  great  majority  of  fossils  are  of  marine  origin, 
most  of  the  sedimentary  formations  in  which  they  occur 
having  been  deposited  upon  the  floor  of  the  sea.  Fresh- 
water and  terrestrial  accumulations  form  an  inconsiderable 
proportion  of  the  series  of  stratified  rocks,  so  that  relics  of 
the  occupants  of  former  rivers,  lakes,  and  dry  lands  are  of 
relatively  infrequent  occurrence.  At  the  present  day  aquatic 
animals  largely  exceed  terrestrial  animals  in  number,  and  the 
same  was  the  case  in  earlier  ages.  On  the  other  hand,  in 
the  world  of  to-day  plants  are  mostly  terrestrial  forms,  and 
although  we  know  very  little  of  the  land-plants  of  the  earlier 
geological  periods,  there  is  no  reason  to  doubt  that  terrestrial 
floras  have,  for  unnumbered  ages,  greatly  surpassed  marine 
floras  in  abundance  and  variety.  The  conditions  for  the 
accumulation  and  preservation  of  plants  in  the  still  waters 
of  lakes,  lagoons,  and  estuaries,  are  upon  the  whole  more 
favourable  than  those  that  obtain  upon  the  sea-floor.  More- 
over, many  seaweeds,  with  their  loose,  cellular  tissues,  are  more 
readily  decomposed  than  the  great  majority  of  the  land- 
plants  with  their  more  enduring  vascular  tissues.  For  these 
and  other  reasons  terrestrial  plants  occur  in  places  more 
abundantly  and  in  a  better  state  of  preservation  than  sea- 
weeds. Nevertheless,  impressions  and  casts  of  the  latter  are 
not  uncommon  in  strata  of  all  ages,  and  hence  it  may  be  said 
that  seaweeds  are  more  widely  distributed  as  fossils  than 
land-plants. 

It  is  obvious,  therefore,  that  from  a  general  point  of  view, 
marine  organic  remains  are  of  most  importance  to  the  student 
of  historical  geology.  It  is  unquestionable  that  the  records 
of  past  times  are  preserved  chiefly  in  the  marine  formations 
of  the  earth's  crust.  It  is  by  studying  these  records  that 
we  are  able  to  follow  the  main  lines  along  which  the  world's 
development  has  taken  place.  The  histories  revealed  by 
freshwater  and  terrestrial  accumulations  are,  as  it  were,  only 
episodes,  although  these  episodes  are  usually  most  interesting 
and  instructive.  Now  and  again,  indeed,  they  may  be  said 
to  constitute  more  or  less  complete  chapters  of  the  general 
world-history.  They  tell  us  of  the  life  of  the  land,  of  which 


96  STRUCTURAL  AND  FIELD  GEOLOGY 

only  sparse  traces  are  met  with  in  marine  formations.  It 
is  obvious,  indeed,  that  the  great  majority  of  land-plants 
and  animals  must  necessarily  disappear  without  leaving  any 
trace  behind.  The  surface  on  which  they  live  is  pre-eminently 
a  region  of  disintegration  and  denudation  rather  than  accumu- 
lation. It  is,  therefore,  only  under  exceptional  circumstances 
that  relics  of  land-life  can  be  preserved.  The  sea,  on  the 
other  hand,  is  par  excellence  the  region  of  accumulation.  The 
creatures  which  live  and  die  there  are  thus  much  more 
likely  to  be  represented.  It  is  simply  for  this  reason  that 
the  records  of  marine  life  are  so  much  more  continuous  and 
abundant  than  those  of  land-life.  Hence  relics  of  land-plants 
and  animals  are,  generally  speaking,  of  relatively  less  value 
to  the  geologist  for  the  purpose  of  comparing  and  correlating 
separate  areas  of  fossiliferous  strata.  Nevertheless,  some  of 
the  most  absorbingly  interesting  and  fascinating  chapters  in 
the  world's  history  have  been  rescued  from  terrestrial  and 
lacustrine  formations.  It  must  also  be  noted  that  in  certain 
cases  it  has  been  possible  to  correlate  widely  separated  areas 
of  terrestrial  and  freshwater  deposits  by  means  of  their 
fossils.  This  holds  specially  true  for  certain  systems  and 
stages,  as  in  the  case  of  the  Coal-measures,  the  coals  and 
lignites  of  later  age,  and  the  estuarine  and  freshwater  deposits 
of  Secondary,  Tertiary,  and  more  recent  times. 

A  glance  at  the  several  great  phyla  of  the  animal  kingdom  will  serve 
to  show  the  relative  importance  to  geologists  of  marine  organic  remains. 

Protozoa. — Among  these  lowly  organised  forms  are  many  which 
possess  calcareous  or  siliceous  hard  parts.  Members  of  this  phylum 
therefore  occur  in  great  abundance  in  marine  formations  of  all  ages. 

Port/era  (Sponges). — A  few  of  the  living  types  are  of  freshwater 
habitat,  but  the  great  majority  are  marine,  and  the  same  was  the  case  in 
earlier  ages.  As  many  sponges  are  furnished  with  a  calcareous  or  a 
siliceous  skeleton  or  framework,  they  are  somewhat  common  fossils  in 
many  marine  deposits. 

Coslenterata. — These  also  are  essentially  marine.  The  phylum 
includes  the  corals,  which  are  among  the  most  abundant  fossils — often 
forming  immense  sheets  and  masses  of  limestone. 

Echinodermata. — This  is  another  great  division  of  marine  creatures, 
amongst  which  are  star-fishes,  sea-urchins,  and  stone  lilies — the  cal- 
careous tests  and  skeletal  remains  of  which  are  among  the  most  frequently 
occurring  fossils  in  many  formations. 

Annelida  (Worms). — These  are  known  as  fossils  chiefly  by  their  tracks 
and  castings — being  for  the  most  part  soft-bodied  creatures,  they  have 


FOSSILS  97 

rarely  been  preserved.  As  these  tracks  and  castings  occur  chiefly  in 
marine  sedimentary  rocks,  it  is  very  doubtful  if  any  of  them  indicate 
earthworms.  The  "tubes"  formed  by  many  marine  annelids  are  often 
met  with  as  fossils. 

Molluscoidea. — These  are  among  the  commonest  and  most  abundant 
fossils.  One  great  division  (Polyzoa)  comprises  the  lace-corals  and  sea- 
mats,  which  are  chiefly  marine,  and,  as  fossils,  often  occur  associated 
with  other  marine  organisms.  The  other  great  division  (Brachiopoda) 
is  exclusively  marine,  and  includes  the  lamp-shells,  etc. — one  of  the  most 
important  types  of  life  with  which  the  student  of  historical  geology  has  to 
deal. 

Mollusca. — The  same  holds  true  with  the  marine  mollusca,  which  are 
more  or  less  abundantly  represented  in  every  great  system  of  strata. 
Not  only  are  they  of  prime  importance  by  reason  of  their  abundance  as 
regards  genera,  species,  and  individuals,  but  their  shells,  like  those  of  the 
brachiopods,  appear  often  in  a  comparatively  perfect  state  of  preserva- 
tion. Freshwater  shells  and  land-snails  are  of  much  less  frequent 
occurrence  as  fossils. 

Arthropoda. — This  phylum  embraces  lobsters,  crabs,  scorpions, 
spiders,  centipedes,  and  insects,  and  is  of  great  value  to  the  geologist 
— the  crustaceans  more  especially,  for  a  large  proportion  of  these 
being  marine,  they  are  well  represented  by  fossils.  Some  of  the  extinct 
types,  as  Trilobites,  for  example,  are  characteristic  fossils  of  the  older 
geological  systems.  Freshwater  and  terrestrial  forms  are  not  so 
commonly  encountered,  since  they  are  largely  confined  to  freshwater 
deposits  and  to  lignite-  and  coal-bearing  strata. 

Vertebrata. — This  great  phylum  is  most  numerously  represented  by 
marine  fishes.  Marine  types  of  reptiles  and  mammals  also  occur  now 
and  again,  but  with  the  exception  of  the  fishes  vertebrate  remains  of  any 
kind  are  sparingly  met  with.  Remains  of  birds  and  land-mammals  are 
almost  confined,  as  might  have  been  expected,  to  freshwater  and  terrestrial 
accumulations. 

Importance  of  Fossils  in  Geological  Investigations. — It 

need  hardly  be  said  that  the  study  of  fossils  to  the  biologist 
is  of  surpassing  importance.  Such  study,  indeed,  cannot  be 
ignored  by  him  if  he  would  understand  the  life-history  of 
existing  types.  But  it  is  not  with  that  side  of  palaeontological 
inquiry  that  the  practical  or  field-geologist  is  mainly  con- 
cerned. He  values  fossils  chiefly  for  tlie  help  they  yield  him 
in  his  endeavours  to  realise  the  conditions  under  which 
sedimentary  rocks  were  formed,  and  to  ascertain  the  chrono- 
logical sequence  of  the  strata. 

Climatic  Conditions  deduced  from  Fossils. — Individual 
fossils,  if  of  existing  species,  and  occurring  in  situ>  may  give 
valuable  evidence  as  to  former  climatic  conditions.  Two 

G 


98  STRUCTURAL  AND  FIELD  GEOLOGY 

examples  may  be  cited.  Certain  relatively  recent  accumula- 
tions of  calcareous  tufa,  occurring  at  La  Celle  near  Paris, 
have  yielded  well-marked  remains  of  the  Canary  laurel 
(Laurus  Canariensis).  There  is  no  doubt,  therefore,  that 
this  plant  formerly  flourished  in  Northern  France.  It  is  no 
longer  a  native  of  that  country,  however,  its  headquarters 
being  in  the  Canary  Islands,  where  it  is  found  flourishing 
luxuriantly  in  the  woody  regions  with  a  northern  exposure, 
between  a  height  of  1600  feet  and  4800  feet  above  the  sea — 
regions  which  are  nearly  always  enveloped  in  steaming 
vapours,  and  exposed  to  the  heavy  rains  of  winter.  The 
temperature  there  keeps  above  69°  F.  during  the  greater  part 
of  the  year,  rarely  falling  in  the  winter  months  below  59°  or 
60°,  and  only  on  the  coldest  days  reaching  49°.  The  presence, 
therefore,  of  this  variety  of  laurel  in  the  Pleistocene  tufa  of 
La  Celle  shows  that  the  winter  climate  of  Northern  France 
must  formerly  have  been  very  mild.  The  laurel  in  question 
is  most  susceptible  to  cold,  and  as  it  flowers  in  the  winter 
season,  it  is  obvious  that  repeated  frosts,  such  as  are  now 
experienced  in  the  north  of  France,  would  prevent  it  repro- 
ducing its  kind.  Another  and  more  familiar  example  of  the 
important  evidence  which  is  sometimes  afforded  by  fossil 
remains  of  existing  types  is  that  of  the  Polar  willow  {Salix 
polar  is) — a  characteristic  arctic  plant,  living  in  Northern 
Lapland,  Spitzbergen,  etc.  This  dwarf  willow  has  been  met 
with  again  and  again  in  Pleistocene  deposits  in  Southern 
Sweden,  Denmark,  England,  etc.,  and  in  various  parts  of 
Central  Europe,  as  far  south  as  Bavaria  and  the  low-lying 
parts  of  Switzerland.  It  cannot  be  doubted,  therefore,  that 
the  appearance  of  the  Polar  willow  so  far  south  of  its  present 
habitat,  points  to  a  very  considerable  climatic  change — arctic 
conditions  would  seem  to  have  prevailed  at  a  relatively  recent 
period  in  what  are  now  the  temperate  latitudes  of  our 
continent. 

It  is  obvious,  however,  that  the  evidence  of  fossils  as  to 
climatic  conditions  must  be  much  stronger  when  a  whole 
assemblage  of  organic  remains  tells  the  same  tale.  In  the 
case  of  the  tufa  of  La  Celle,  for  example,  the  Canary  laurel 
is  accompanied  by  the  remains  of  many  other  plants,  as 
well  as  by  shells  of  land-snails,  each  of  which  is  indicative 


FOSSILS  99 

of  a  milder  and  more  equable  climate  than  now  characterises 
Northern  France.  And  the  same  is  the  case  with  the  Polar 
willow,  the  evidence  supplied  by  it  being  fortified  by  that 
of  other  high  northern  plants,  and  by  the  relics  of  such  animals 
as  lemming,  arctic  fox,  etc. 

Great  caution  must  be  exercised  in  deducing  climatic 
conditions  from  the  occurrence  of  extinct  forms  of  life.  For 
these,  even  when  they  very  closely  resemble  living  types, 
need  not  have  existed  under  similar  conditions.  For  example, 
so  long  as  the  mammoth  and  woolly  rhinoceros  were  only 
known  from  their  skeletal  remains,  they  were  generally 
supposed  to  have  existed  under  the  same  climatic  conditions 
as  their  living  representatives.  We  know  now,  however,  that 
each  was  provided  with  a  thick  woolly  and  hairy  covering, 
and  was  capable,  therefore,  of  withstanding  the  rigours  of  a 
northern  winter. 

In  dealing  with  fossils  consisting  largely  of  extinct  species, 
it  is  the  general  facies  of  a  flora  and  fauna,  and  not  individual 
forms,  that  are  to  be  specially  considered.  For  example,  the 
London  Clay  (Eocene)  has  yielded  a  large  number  of  types 
having  a  tropical  or  subtropical  aspect.  Amongst  the  plants 
are  forms  of  sarsaparilla,  aloe,  amomum,  fan-palms,  fig,  liquid- 
ambar,  magnolia,  eucalyptus,  cinnamon,  various  proteaceous 
plants,  etc. ;  while  the  animals  include  turtles,  tortoises, 
crocodiles,  tapir-like  pachyderms,  and  certain  birds  with 
affinities  to  living  tropical  types.  Associated  with  these  are 
many  forms  of  molluscan  life  which  have  their  nearest  living 
representatives  in  warm  latitudes,  such  as  cones,  cowries, 
volutes,  nautilus,  etc.,  together  with  sword-fish,  saw-fish, 
sharks,  and  rays.  All  this  is  good  evidence  that  a  warm 
climate  prevailed  during  the  deposition  of  the  London  Clay. 
The  land  was  clothed  with  a  tropical  or  subtropical  vegetation, 
while  analogous  types  of  animal-life  haunted  the  rivers  and 
flourished  in  the  sea  of  the  period. 

In  the  older  geological  systems  we  may  say  that  all  the 
species  and  nearly  all  the  genera  are  extinct,  so  that  any 
general  resemblance  which  an  assemblage  of  Palaeozoic  fossils 
may  have  to  those  of  some  particular  groups  of  living  plants 
and  animals  may  have  no  climatic  significance  whatsoever. 
We  may  feel  sure,  indeed,  that  the  abundant  flora  of  the 


100  STRUCTURAL  AND  FIELD  GEOLOGY 

Carboniferous  period  could  not  have  flourished  under  arctic 
or  even  cold  temperate  conditions  of  climate ;  and  we  may  oe 
equally  convinced  that  the  abundant  corals  and  cephalopods 
of  Palaeozoic  times,  with  their  numerous  congeners,  were  not 
denizens  of  cold  seas.  Existing  conditions  might  even  lead 
us  to  believe  that  the  massive  limestones  of  those  early  ages 
were  most  likely  formed  in  genial  waters.  For  at  the  present 
day  it  is  in  warm  seas  that  lime-secreting  organisms,  such 
as  corals,  pelagic  molluscs,  and  foraminifera,  flourish  most 
abundantly,  and  are  there  giving  rise  to  widespread  and  thick 
accumulations  of  calcareous  matter.  But  whether  the  climatic 
conditions  of  Palaeozoic  times  were  similar  to  those  of  our 
present  warm  latitudes,  it  would  be  rash  to  conclude. 

When  the  geographical  distribution  of  Palaeozoic  floras  and 
faunas,  however,  is  kept  in  view,  we  may  advance  our 
inferences  a  step  further.  Should  the  fossils  or  groups  of 
fossils  of  some  particular  formation  be  known  to  occur  over 
vast  areas  of  the  earth's  surface,  in  arctic,  temperate,  sub- 
tropical, and  tropical  latitudes,  and  even  in  similar  latitudes 
of  the  Southern  Hemisphere,  we  should  be  justified  in  the 
inference  that  the  climatic  conditions  indicated  by  the  fossils 
in  question  must  have  been  singularly  equable.  The  mere  fact 
that  in  the  earlier  stages  of  the  world's  history,  cosmopolitan 
forms  of  plant-  and  animal-life  abounded,  affords  good  ground 
for  believing  that  the  climatic  conditions  of  those  far  past 
times  differed  considerably  from  the  present.  The  climate  of 
the  globe  in  those  days  could  not  have  been  differentiated 
into  such  distinct  zones  as  is  now  the  case. 

Geographical  Conditions  deduced  from  Fossils. — Fossils 
naturally  yield  evidence  as  to  terrestrial,  freshwater,  and 
marine  conditions. 

(a)  Land-surfaces. — These  are  seldom  preserved.  Never- 
theless, they  do  occur  in  strata  belonging  to  widely  separatee! 
periods.  Now  and  again,  for  example,  the  stools  and  roots 
of  trees  penetrating  ancient  soils,  occur  interbedded  with 
sedimentary  strata,  a  good  example  being  furnished  by  the 
"  dirt-bed "  of  Portland.  This  dirt-bed  is  simply  an  old  soil 
containing  the  roots  and  stumps  of  extinct  forms  of  cycads 
and  conifers.  It  is  intercalated  between  beds  of  freshwater 
origin,  a  succession  which  shows  that,  after  the  deposition  of 


FOSSILS  islW- 

a  wide  area  of  fluviatile  mud,  dry  land  prevailed  and  eventually 
Became  covered  with  forests.  .  Subsequently,  owing  probably 
to  subsidence,  the  forest  was  submerged  and  buried  under 
newer  accumulations  of  fluviatile  mud  and  silt.  Many  of  the 
coal-seams  of  the  Carboniferous  period,  with  their  underclays, 
tell  a  similar  tale,  and  the  same  history  is  repeated  by  not 
a  few  of  the  lignites  belonging  to  later  geological  periods. 
[Many  coals  and  lignites,  however,  appear  to  represent  masses 
of  vegetable  matter  which  have  probably  been  drifted  from 
the  land  into  estuaries  and  shallow  bays  of  the  sea.]  The 
not  infrequent  occurrence  of  arachnids,  insects,  lizards,  and 
land-snails  associated  with  beds  of  coal  and  lignite,  is  additional 
evidence  of  terrestrial  conditions.  Amber,  again,  is  an 
abundant  product  of  the  lignite-bearing  beds  of  Germany, 
and  unquestionably  represents  the  gum  and  resin  which 
exuded  from  some  of  the  forest  trees  of  Tertiary  times. 

(b)  Lacustrine  conditions. — These   are    indicated    by   the 
presence     of     numerous     freshwater     molluscs     and     small 
crustaceans  which   are   sometimes   so   abundant   as   to   form 
beds   of    marl   and   limestone.      Plant-remains,   insects,   and 
other  relics  of  land-life,  such  as  reptiles  or   mammals,  often 
occur   in    lacustrine    deposits.      It    is    from    lacustrine    and 
estuarine  deposits,  indeed,  that  we  obtain  our  fullest  informa- 
tion as  to  the  life  of  former  land-surfaces. 

(c)  Marine  conditions. — Relatively  deep  or,  at  least,  clear 
water  is  indicated  by  thick  masses  of  limestone,  more  or  less 
abundantly  charged  with  corals,  sea-lilies,  and  other  marine 
organisms.     This  inference  is  based  partly  on  the  fact   that 
these     limestones     are    comparatively    pure — that    is,    they 
contain  relatively  little   insoluble  matter,  and  this  is  usually 
in  a  very  finely  divided  state.     In  short,  it   is   evident   that 
such  limestones  have  accumulated  over  parts  of  the  sea-floor 
not  reached   by  ordinary  sediment — conditions  which,  as   a 
rule,  can   obtain   only  at   a  considerable   distance   from   the 
shore,  and  often,  therefore,  in  somewhat  deep  water.     Further, 
we  judge  from  the  analogy  of  the  present,  that,  as  existing 
corals  only  flourish  in  clear  water,  their  predecessors  probably 
demanded    similar    conditions.      This    inference    is     further 
strengthened   by  the   fact  that  when,  towards   the  top   of  a 

of  limestone,  the  rock  becomes  more  and  more  impure. 


STRUCTURAL  AND  FIELD  GEOLOGY 

the  corals,  and  certain  of  their  congeners,  often  begin  to 
diminish  in  size,  and  even  to  become  somewhat  distorted,  as 
if  the  influx  of  muddy  sediment  had  acted  prejudicially  upon 
their  growth  and  development. 

Shallow-water  conditions  and  proximity  of  the  land  are 
often  evidenced  by  trails,  burrows,  and  castings  of  annelids, 
tracks  of  crustaceans,  etc.,  footprints  of  reptiles,  amphibians, 
birds,  or  mammals.  Along  with  these  the  strata  may  yield 
more  or  less  well-preserved  plants,  insect-remains,  and  other 
relics  of  land-life.  Beds  containing  such  fossils  are  not 
infrequently  estuarine  deposits,  and  often  exhibit  ripple- 
marks,  rain-prints,  and  sun-cracks. 

Terrestrial  Movements  deduced  from  Fossils. — The 
presence  of  marine  fossils  in  a  rock  obviously  indicates 
oscillations  of  the  sea-level.  The  appearance,  for  example, 
in  our  maritime  districts,  at  various  heights  above  the  present 
sea-level,  of  terraces  of  sand  and  gravel,  crowded  with  sea- 
shells  of  still  living  species,  is  proof  positive  of  some  recent 
crustal  movement — either  the  land  has  risen  or  the  sea-floor 
has  subsided.  Again,  the  existence  at  various  depths  on  the 
sea-bottom  of  peat  overlying  the  stools  of  trees  belonging 
to  kinds  that  still  flourish  in  these  islands,  is  evidence 
sufficient  of  a  recent  subsidence  of  the  land. 

Geological  Chronology  and  Fossils.— In  many  cases 
it  is  quite  impossible  to  correlate  the  formations  occurring  in 
separate  regions  by  means  of  lithological  characters  alone. 
Within  limited  areas  these  may  be  reliable,  but  strata  begin 
to  change  in  character  as  they  extend  in  various  directions. 
Limestones,  for  example,  may  become  gradually  more  and 
more  argillaceous  until  at  last  they  merge  into  shales,  while 
these  last  may  in  their  turn  eventually  pass  into  or  inter- 
osculate  with  sandstones.  Now,  unless  such  changes  could 
be  followed  in  continuous  open  section,  we  could  not  possibly 
be  sure  that  certain  given  beds  of  limestone,  shale,  and  sand- 
stone were  exactly  contemporaneous — all  laid  down  on  one 
and  the  same  sea-floor.  These  rocks  are  so  dissimilar  that, 
unless  we  actually  traced  the  connecting  passages,  we  could 
not  tell  how  one  was  related  to  another.  So  far  as  lithological 
character  is  concerned,  they  might  each  have  been  formed 
at  a  different  time,  But  if  the  separate  sections  pf  strata 


FOSSILS  103 

contained  fossils  having  the  same  general  facies — and 
especially  if  several  species  were  common  to  the  limestones, 
the  shales,  and  the  sandstones,  we  could  no  longer  doubt 
that  all  these  rocks  were  accumulations  formed  in  one  and 
the  same  sea.  Fossils  are  thus  of  paramount  importance  in 
the  correlation  of  strata. 

In  the  attempt  to  determine  the  relative  age  of  our 
fossiliferous  strata,  the  most  important  step  was  taken  when 
William  Smith,  the  father  of  Stratigraphical  Geology,  deter- 
mined the  sequence  of  the  Mesozoic  strata  of  England,  and 
ascertained  that  each  subdivision  of  that  great  series  of  rocks 
was  characterised  by  the  presence  of  certain  particular  types 
of  fossils.  Following  his  lead,  geologists  have  since  estab- 
lished the  Stratigraphical  succession  of  fossiliferous  strata 
throughout  the  major  portion  of  the  world.  It  is  now 
recognised  that  every  well-defined  formation  is  marked  by 
the  presence  of  a  particular  flora  or  fauna,  or  by  certain 
genera  and  species  which  are  restricted  to  it.  The  presence 
of  these  type-fossils^  as  they  are  termed,  enables  the  geologist 
at  once  to  assign  the  rocks  in  which  they  occur  to  some 
definite  horizon  or  stage  in  the  great  succession  of  sedi- 
mentary accumulations. 

It  is  obvious,  therefore,  that  some  knowledge  of  type- 
fossils  must  be  of  great  use  in  Practical  Geology.  How 
greatly  they  help  a  geologist  in  his  endeavour  to  work  out 
a  Stratigraphical  succession  will  be  shown  when  the  subject  of 
geological  surveying  comes  to  be  discussed. 


CHAPTER  VII 

STRATIFICATION   AND  THE   FORMATION   OF   ROCK-BEDS 

Consolidation  of  Incoherent  Accumulations.  Lamination  and  Strati- 
fication. Extent  and  Termination  of  Beds.  Contemporaneous 
Erosion.  Grouping  of  Strata.  Contemporaneity  of  Strongly  Con- 
trasted Strata.  Diagonal  Lamination  and  Stratification.  Surface- 
markings. 

TECTONIC  or  Structural  Geology  treats  of  the  arrangement 
of  rocks,  or  the  mode  of  their  occurrence.  It  deals,  in  short, 
with  the  architecture  of  the  earth's  crust.  The  study  of 
rocks,  Petrography,  is  concerned  simply  with  the  nature  and 
origin  of  rocks  as  aggregates  of  mineral  matter.  For  the 
purposes  of  geology,  however,  this  is  not  sufficient — rocks  must 
be  studied  not  only  in  hand  specimens,  but  as  constituents  of 
the  earth's  crust.  The  geologist  must  take  note  of  the 
positions  they  occupy  as  rock-masses,  and  the  relation  which 
the  various  rock-masses  bear  to  one  another.  It  is  only  by 
such  observations  that  the  order  of  succession,  or,  in  other 
words,  the  relative  age  of  rock-masses,  can  be  ascertained, 
and  the  particular  conditions  under  which  they  were  formed, 
and  the  various  changes  they  have  since  undergone,  can  be 
determined.  It  is,  therefore,  hardly  too  much  to  say  that  our 
knowledge  of  the  many  revolutions  which  have  affected  the 
earth's  surface — the  ever-changing  geographical  conditions  of 
the  past — is  largely  based  on  the  study  of  structural  geology. 
In  discussing  the  important  subject  to  which  the  following 
chapters  are  devoted,  attention  must  be  largely  confined  to  a 
description  of  rock-structures ;  but  it  may  be  helpful  to  the 
student  now  and  again  to  consider  what  such  structures 
mean,  and  to  show  how  they  may  be  interpreted  by  reference 
to  existing  operations  of  nature, 

104 


STRATIFICATION  105 

The  structural  geology  of  derivative  rocks  is  upon  the 
whole  simpler  and  more  readily  understood  than  that  of 
eruptive  and  metamorphic  rocks,  and,  therefore,  we  shall 
consider  first  the  phenomena  which  are  specially  characteristic 
of  aqueous  or  sedimentary  deposits.  As  we  shall  learn  in 
the  sequel,  however,  many  of  the  structures  presently  to  be 
described  are  met  with  likewise  among  igneous,  and  some  of 
them  even  among  metamorphic,  rocks. 

Consolidation  of  Incoherent  Accumulations. — By  way 
of  introduction  to  the  present  subject,  a  few  remarks  on  the 
consolidation  of  rocks  may  not  be  out  of  place.  Rocks,  as 
we  have  learned,  are  not  all  equally  compacted,  and  their 
state  of  solidification  is  no  certain  test  of  their  relative 
age.  It  holds  generally  true,  however,  that  the  fragmental 
accumulations  of  early  geological  ages  are  more  consolidated 
than  those  which  have  been  formed  in  later  times.  We 
cannot  doubt  that  conglomerates,  sandstones,  shales,  lime- 
stones, tuffs,  and  volcanic  agglomerates  were  formerly  as 
loose  and  incoherent  as  any  similar  masses  now  in  course  of 
formation.  There  is  one  obvious  way  in  which  some  of  these 
accumulations  have  become  hardened,  and  we  can  see  the 
process  in  operation  at  the  present  day.  Water  percolating 
through  loose  sand  and  gravel  introduces  mineral  matter 
which,  as  the  water  evaporates,  is  deposited  between  the 
grains  and  pebbles,  and  thus  binds  these  together.  Frequently 
a  sediment  becomes  compacted  by  the  chemical  action  of 
water  upon  its  own  constituents.  Calcareous  accumulations, 
for  example,  tend  to  become  consolidated  by  the  solution  of 
the  calcium-carbonate,  and  its  subsequent  precipitation  in 
pores  and  interstices ;  what  were  formerly  yielding  incoherent 
masses  becoming  in  this  way  converted  into  hard  rocks,  such 
as  calcareous  sandstones,  grits,  and  limestones.  We  know 
also  that  loose  or  soft  materials  may  be  compacted  by  the 
weight  of  overlying  masses.  Peat,  for  example,  taken  from 
the  bottom  of  a  bog,  some  twenty  or  thirty  feet  in  depth,  is 
often  so  compacted  that  when  dried  it  resembles  lignite.  In 
like  manner,  thick  artificial  accumulations  of  loose  rock-rubbish, 
as  everyone  knows,  become  in  time  sufficiently  consolidated 
to  serve  as  foundations  for  buildings.  When  we  are  assured, 
therefore,  that  many  rocks  of  sedimentary  origin,  now  visible 


106  STRUCTURAL  AND  FIELD  GEOLOGY 

at  the  surface,  were  formerly  overlaid  by  hundreds  or  even 
thousands  of  feet  of  younger  strata  which  have  since  been 
removed   by  the  gradual  process  of  denudation,  we  cannot 
doubt  that  the  mere  weight  of  such  enormous  masses  must 
have  tended  to  consolidate  the  beds  upon  which  they  rested. 
Once  more,  we  note  that  heat  tends  to  solidify  deposits,  as 
may  be  seen  in  the  case  of  strata  which  have  been  baked  and 
hardened  by  intrusions  of  formerly  molten  matter — eruptive 
masses.     More  potent   and  widespread,  however,  must   have 
been  the  action  of  the  internal  heat  of  the  globe  upon  thick 
accumulations  of  sediment  deposited  during  long-continued 
subsidence   of  the   sea-floor.     Certain   consecutive   series  of 
strata  attain  a  thickness  of  15,000  or  20,000  feet  and  more. 
It  is  obvious,  therefore,  that  while  the  upper  members  of  such 
series  were  being  accumulated,  the  lower  members  must  have 
been  more  or  less  affected  by  the  rise  of  the  isogeotherms  or 
lines  of  equal  subterranean  temperature.     According  to  what 
is  known  of  the  increment  of  heat  downwards,  a  very  high 
temperature  must  obtain  at  depths  of  15,000  or  20,000  feet 
from  the  surface — certainly  much  in  excess  of  the  boiling- 
point    of   water.     Strata    brought    in    this    way   under    the 
influence  of  the  internal  heat  of  the  globe  could  hardly  escape 
some  degree  of  change.     Not  only  would  they  be  compressed 
by  the  superincumbent  masses,  but  if  interstitial  water  were 
present,   chemical   reactions   amongst   the   various   rock-con- 
stituents might  often  be  greatly  stimulated — water  acting  as 
a  more  powerful  agent  under  increased  heat  and  pressure.     It 
can  hardly  be  doubted  that  such  must  have  been  among  the 
chief   causes   of    the   consolidation    of   ancient    sedimentary 
accumulations. 

Pressure  may  be  brought  about,  however,  by  other  means 
than  the  mere  weight  of  overlying  masses.  The  earth  is  a 
cooling  body,  and  as  the  crust  sinks  down  upon  the  slowly 
contracting  nucleus,  it  necessarily  becomes  subject  to  enormous 
lateral  compression.  To  this  it  can  only  yield  by  folding  and 
crumpling  up,  and  thus  the  rocks  of  which  it  is  composed  are 
frequently  more  or  less  highly  disturbed.  Strata  which 
originally  occupied  approximately  horizontal  positions  are 
now  flexed,  bent,  and  inclined  at  all  angles,  and  such  highly 
disturbed  rocks,  no  matter  what  their  geological  age  may  be, 


STRATIFICATION  107 

are  invariably  much  compacted,  and  sometimes  so  altered 
as  to  become  truly  .crystalline  and  schistose.  On  the  other 
hand,  strata  which  have  not  been  disturbed,  but  still  retain 
their  original  horizontal  position,  are  usually  much  less 
hardened,  and  rarely  show  any  trace  of  metamorphism.  The 
older  Palaeozoic  rocks  of  this  country,  for  example,  are 
usually  highly  flexed  and  folded,  and  not  only  much  com- 
pressed and  hardened,  but  frequently  rendered  crystalline 
and  schistose.  In  Central  Russia,  on  the  other  hand,  strata 
of  the  same  age  have  retained  their  original  horizontal 
position,  and  are  so  unaltered  in  general  aspect  as  to  resemble 
the  sedimentary  accumulations  of  comparatively  recent  times. 
The  contrast  between  the  undisturbed  and  more  or  less 
incoherent  Eocene  deposits  of  the  London  Basin  and  their 
much  flexed  and  folded  representatives  in  the  Alps  of 
Switzerland,  is  not  less  striking.  Many  similar  contrasts 
might  be  cited,  but  it  is  enough  to  emphasise  the  fact 
that  great  crustal  deformation  is  invariably  accompanied 
by  the  induration  of  the  rocks  affected,  no  matter  what 
their  age  may  be.  We  may  conclude,  therefore,  that 
the  pressure  induced  by  crustal  movements  has  been 
one  of  the  most  effectual  and  widely  acting  causes  of  rock- 
consolidation. 

Lamination  and  Stratification. — The  most  abundant  and 
widely  distributed  rocks  of  derivative  origin  are  undoubtedly 
the  sedimentary  types,  conglomerate,  sandstone,  and  shale. 
They  have  been  spread  out  by  the  sorting  action  of  water, 
and  consequently  occur  in  sheet-like  form.  Coarse  gravel 
(conglomerate)  has  obviously  been  deposited  upon  beaches, 
or  in  shallow  water  at  the  mouths  of  rapid  torrents,  streams, 
and  rivers.  It  may  therefore  be  fluviatile,  lacustrine,  estuarine, 
or  marine.  In  like  manner  sand  (sandstone]  and  argillaceous 
sediments  (clay,  shale,  etc.)  are  of  both  marine  and  freshwater 
origin.  Sometimes  a  sedimentary  rock  has  been  deposited 
more  or  less  rapidly ;  in  other  cases  the  process  of  sedimenta- 
tion has  been  gradual  and  protracted.  In  the  latter  case, 
this  is  shown  by  the  structure  of  the  sheet-like  deposit,  which 
is  usually  composed  of  successive  layers  or  very  thin  laminae. 
In  a  deposit  more  rapidly  accumulated,  this  structure  is  either 
inconspicuous  or  wanting. 


108 


STRUCTURAL  AND  FIELD  GEOLOGY 


Lamination  is  typically  represented  by  the  finer  grained 
sediments,  such  as  argillaceous  shales.  The  laminae  of  such 
deposits  vary  in  thickness  from  an  inch  or  so  clown  to  the 
finest  films,  not  thicker  than  ordinary  writing-paper  (see 
Fig.  4).  As  a  rule  they  cohere  only  slightly,  so  that  a  rock 
of  the  kind  is  more  or  less  readily  separated  along  the  planes 
of  lamination.  Not  infrequently,  however,  the  laminae  have, 
owing  to  pressure,  become  more  adherent.  The  laminated 
structure  being  the  result  of  successive  depositions  of  fine 
sediment  by  periodical  river-floods,  or  by  tidal  or  other 
marine  currents,  usually  indicates  accumulation  in  quiet  water. 
These  conditions  are  met  with  in  lakes  and  estuaries,  and  over 
such  areas  of  the  sea-floor  as  are  not  much  disturbed  by 

currents — that  is  to  say, 
in  relatively  deep  water. 
Although  lamination  is 
very  characteristic  of 
argillaceous  rocks,  it  is 
by  no  means  confined 
to  these.  Laminated 
sandstones  are  of  com- 
mon occurrence,  particu- 
larly when  the  rock  is 
very  fine-grained  and 
more  or  less  argillaceous. 
In  coarser  grained  sand- 
stones the  individual 
laminae  are  thicker  than  in  argillaceous  shales.  When  they 
exceed  an  inch  or  so,  however,  they  are  often  described  as 
layers. 

Bed  or  Stratum  is  the  term  applied  to  any  sheet-like 
mass  which  has  a  more  or  less  definite  petrographical 
character,  and  is  separated  by  well-marked  parallel  division- 
planes  from  overlying  and  underlying  rocks.  A  bed  may  be 
homogeneous  and  without  any  apparent  arrangement  of  its 
constituents,  or  it  may  consist  of  successive  layers  or  laminae. 
It  is  well  to  point  out,  however,  that  the  terms  "bed"  or 
"  stratum "  and  "  layer "  are  purely  relative.  A  sandstone 
consisting  of  a  series  of  layers,  for  example,  is  often  described 
as  a.  thin-bedded  rock.  Again,  a  thin  sheet  of  limestone,  iron- 


FIG.  4. — STRATIFICATION  AND 
LAMINATION. 

s, s,  s,  non-laminated  beds ;  I,  I,  laminated  beds. 


STRATIFICATION  109 

stone,  or   coal,  intercalated  in  a  series  of  shales,  might  be 
termed  either  a  bed,  a  layer,  or  a  seam. 

The  time  required  for  the  formation  of  any  given  thickness  of  sedi- 
mentary materials  is  necessarily  indeterminate.  Generally  speaking, 
however,  a  bed  of  conglomerate  may  have  been  amassed  more  rapidly 
than  an  equal  thickness  of  sandstone,  and  a  sheet  of  sandstone  may 
have  been  deposited  in  a  shorter  time  than  one  of  shale  of  equivalent 
extent  and  thickness.  It  is  clear,  however,  that  the  rate  of  deposition 
of  any  particular  kind  of  sediment  must  vary  indefinitely.  Certain 
sandstones,  for  example,  may  have  been  formed  more  rapidly  than 
others  of  precisely  the  same  character.  Usually,  however,  where  the 
rate  of  accumulation  has  varied  in  any  marked  degree,  some  evidence  of 
this  will  be  visible  in  the  structure  of  the  rocks.  Thus,  we  may  reasonably 
infer  that  a  homogeneous  sandstone,  such  as  freestone  or  liver-rock,  has 
been  formed  in  less  time  than  an  equal  mass  of  laminated  sandstone. 
The  liver-rock  indicates  continuous  sedimentation,  while  the  laminated 
sandstone  points  to  a  process  of  intermittent  sedimentation.  So,  again, 
a  structureless  clay  or  loam  has  probably  been  accumulated  more 
continuously,  and  therefore  more  rapidly,  than  a  well-laminated  shale. 
Nevertheless,  it  must  be  admitted  that,  in  comparing  separate  beds  of 
similar  character  and  thickness,  we  can  never  be  sure  that  an  equal  time 
was  required  for  their  deposition.  Nay,  even  in  the  case  of  beds  having 
the  same  composition  and  structure,  and  differing  only  in  thickness,  it 
cannot  always  be  assumed  that  the  thickest  beds  took  the  longest  time 
for  their  accumulation.  Probably,  in  most  cases,  they  did,  but  many 
facts  conspire  to  show  that  mere  thickness  is  no  sure  test  of  the  relative 
age  of  individual  beds.  If  this  be  true  of  strata  having  the  same 
character  throughout,  it  is  certainly  not  less  true  of  beds  which  differ  in 
composition  and  structure.  A  series  of  limestones  and  shales,  one 
hundred  feet  in  thickness,  for  example,  may  well  have  required  for  its 
formation  a  far  longer  time  than  a  succession  of  several  thousand  feet  of 
sandstones. 

Intervals  indicated  by  Planes  of  Lamination  and  Stratification. — The 
parallel  division-planes  separating  individual  strata  are  always  more  pro- 
nounced than  planes  of  lamination,  i.e.  the  planes  separating  individual 
layers  or  laminae.  This  naturally  suggests  that  a  longer  time  has  elapsed 
between  the  accumulation  of  successive  strata  than  between  the  deposition 
of  successive  laminae  or  layers.  The  length  of  interval  represented  by 
planes  of  stratification,  however,  is  indeterminate.  It  may  be  quite 
short  or  very  prolonged.  In  the  case  of  shallow- water  sediments,  which 
are  apt  to  show  rapid  alternations  of  coarser  and  finer  grained  deposits, 
no  long  intervals  need  have  separated  the  deposition  of  the  several  kinds 
of  sediment  from  each  other.  Rapid  alternations  of  sediment  are  quite 
characteristic  of  alluvial,  estuarine,  and  littoral  or  shore-accumulations. 
On  the  other  hand,  sediments  accumulated  in  deeper  water  seldom  show 
such  rapid  changes  of  character.  They  are  usually  fine-grained  and 
persistent  over  wide  areas.  It  is  justifiable,  therefore,  to  infer  that  planes 


110 


STRUCTURAL  AND  FIELD  GEOLOGY 


of  stratification  amongst  such  accumulations  will  represent  longer 
intervals  than  in  the  case  of  estuarine  and  littoral  deposits.  Should  a 
pure  marine  limestone  of  some  thickness,  for  example,  be  immediately 
underlaid  and  overlaid  by  thick  argillaceous  shales,  as  in  the  accompanying 
illustration  (Fig.  5),  we  should  be  justified  in  assuming  that  the  planes 
of  stratification  indicated  lengthy  intervals  of  time.  Such  an  alternation 
of  deposits  would  necessarily  imply  certain  geographical  changes,  and 
these,  as  a  rule,  are  only  developed  very  slowly.  We  should  infer  that 
some  change  of  conditions  had  arrested  the  deposition  of  muddy 
sediment  represented  by  the  lower  beds  of  shale  (skl] — either  the  source 

of  supply  was  cut  off,  or  the 
current  which  brought  the  sedi- 
ment had  lost  its  force,  or  was 
diverted  in  some  other  direc- 
tion. The  presence  of  thick  pure 
limestone  (/),  consisting  of  the 
debris  of  corals  and  other 
marine  organisms,  points  to  a 
FIG.  5. — SHALES  AND  LIMESTONE.  long-continued  period  during 
s/a,  stf,  shales;  I,  limestone.  which  the  water  remained  clear. 

Then  the  sudden  appearance  of 

the  overlying  shales  indicates  a  resumption  of  the  conditions  which 
obtained  during  the  deposition  of  the  lower  shales.  Possibly  the  alter- 
nation of  beds  may  point  to  crustal  movements.  It  may  be  that  the  floor 
of  the  sea  subsided  so  as  to  carry  it  beyond  the  reach  of  mud-transporting 
currents,  and  after  a  prolonged  period  of  rest,  during  which  the  corals  and 
their  congeners  flourished,  a  new  crustal  movement  in  the  opposite  direc- 
tion brought  the  same  region  again  within  the  influence  of  currents  laden 
with  fine  sediment.  Explain  the  alternation  of  strata  as  we  may,  it  is 
obvious  that  the  planes  of  stratification  in  this  case  indicate  more  or  less 
prolonged  intervals.  Geologists  do  not  doubt  that  in  some  cases  these 
planes  may  well  represent  a  longer  period  of  time  than  was  required  for 
the  accumulation  of  the  various  strata  which  they  separate. 

Extent  and  Termination  of  Beds. — Fine-grained  deposits 
usually  have  a  wider  extension  than  coarse-grained  accumula- 
tions. This  is  quite  in  keeping  with  what  we  know  of  the 
distribution  of  sediments  in  the  lakes  and  seas  of  our  own 
day.  When  a  river  enters  a  lake  or  estuary,  the  force  of 
the  current  is  immediately  checked,  and  the  heavier  and 
coarser  materials,  gravel,  etc.,  are  at  once  thrown  down.  Grit 
and  sand  are  swept  out  to  a  greater  distance,  and  more 
extensively  distributed,  while  the  finest  particles  travel  further 
still,  and  are  spread  over  a  yet  wider  area.  Practically  the 
same  kind  of  sifting-out  of  sediments  is  effected  by  waves 
and  tidal  currents  along  an  open  coast-line.  Banks  of  shingle 


STRATIFICATION  111 

and  gravel  accumulate  close  inshore,  while  grit  and  sand  are 
carried  further  off,  and  the  lightest  or  most  readily  trans- 
ported sediment  further  still,  the  finer  deposits  invariably 
extending  over  the  widest  tract  of  sea-floor.  Beds  of  shale, 
therefore,  will  generally  have  a  greater  lateral  extension  than 
beds  of  sandstone,  grit,  and  conglomerate  occurring  in  the 
same  series  of  strata.  Marine  limestones,  even  when  thin, 
often  range  over  a  very  wide  area.  For  their  formation 
somewhat  clear  water  is  required,  and,  unless  they  be  of  the 
nature  of  coral-reefs,  they  will  usually  have  accumulated  at 
some  distance  from  any  land,  and  consequently  often  in 
relatively  deep  water.  Under  such  conditions,  therefore,  we 
might  have  expected  them  to  have  a  wide  extension.  All 
this  is  in  keeping  with  the  broad  fact  that  accumulation  of 
sediments  proceeds  with  least  interruption  over  those  parts 


FIG.  6. — DISTRIBUTION  OF  MARINE  ACCUMULATIONS. 

3,  gravel  and  sand ;  c,  clay,  mud,  etc. ;  o,  organic  accumulations. 

of  the  sea-floor  which  are  not  strongly  swept  by  currents. 
Where  there  is  much  stir  in  the  waters  deposition  of  sediment 
is  frequently  interrupted,  the  sediments  are  coarse-grained, 
and  show  constant  alternations  of  gravel,  grit,  and  coarse 
sand.  Where  the  sea-floor  is  not  so  liable  to  the  scouring 
action  of  tidal  currents,  finer  sand  is  spread  far  and  wide, 
and  passes  out,  as  greater  depths  are  reached,  into  mud  and 
silt,  which  extend  over  still  wider  tracts  of  undisturbed  sea- 
floor.  At  last  a  zone  is  approached,  beyond  which  little 
or  no  terrigenous  material  is  carried.  Here  the  most  import- 
ant oceanic  accumulations  are  of  organic  origin,  calcareous 
and  siliceous  oozes1  (see  Fig.  6). 

Each  particular  stratum  in  a  sedimentary  series  may  be  looked  upon 
as  a  lenticular  sheet,  which,  seen  in  section,  begins  at  zero,  thickens  out 
regularly  or  irregularly  as  the  case  may  be,  until  it  reaches  its  maximum 
development,  and  then  thins  off  in  the  same  way.  This  lenticular 
structure  can  often  be  seen  in  one  and  the  same  quarry,  where  the  whole 
group  of  beds  may  consist  of  a  series  of  short,  imbricating,  overlapping, 


112  STRUCTURAL  AND  FIELD  GEOLOGY 

and  interosculating  lenticular  sheets.  Thicker  and  more  continuous 
strata  of  sedimentary  origin  behave  in  precisely  the  same  way,  the  beds 
of  coarsest  materials  thickening  out  and  thinning  off  more  rapidly  than 
the  fine-grained  deposits.  Such  being  the  manner  in  which  strata  are 
arranged,  it  is  obvious  that  sections  taken  across  the  same  series  of 
strata  at  different  places  will  not  often  show  the  same  number  of  beds, 
or  if  all  be  present,  they  will  probably  vary  in  thickness.  In  the  following 
section  (Fig.  7),  for  example,  we  have  from  a  to  d  an  apparently  con- 


J. 


FIG.  7. — THINNING-OUT  OF  STRATA. 

secutive  series  of  beds,  and  there  is  nothing  at  that  end  of  the  section 
to  show  that  the  several  separating  planes  of  stratification  do  not 
represent  similar  intervals.  Yet  we  see  that  one  plane  (.r,  .x)  really 
indicates  a  longer  interruption  or  pause  in  the  process  of  sedimentation 
than  the  others,  a  pause  of  sufficient  duration  to  permit  of  the  accumula- 
tion of  the  beds  bracketed  at  2. 

Contemporaneous  Erosion — The  accumulation  of  rela- 
tively shallow-water  deposits  rarely  goes  on  without  interrup- 
tion, for  the  currents  which  transport  and  lay  down  sediment 
not  infrequently  vary  this  action  by  scouring  it  out  again 
and  retransporting  it  elsewhere.  Thus,  during  the  formation 
of  lacustrine,  estuarine,  and  littoral  and  sublittoral  deposits, 
accumulation  and  erosion  often  alternate.  In  the  accompany- 
ing section  (see  Fig.  8)  we  have  a  series  of  beds,  the 
accumulation  of  which  has  obviously  been  arrested  at  intervals. 
The  bottom  stratum,  consisting  of  sandy  clay  (c],  points  to 
deposition  in  relatively  quiet  water.  After  such  conditions 
had  obtained  for  some  time,  the  accumulation  of  fine  sediment 
suddenly  ceased,  and  the  area  of  deposition  was  traversed 
by  a  stronger  current  which  trenched  and  furrowed  the 
stratum  of  sandy  clay.  As  the  force  of  this  current  declined, 
sand  (s)  began  to  be  distributed  over  the  denuded  surface 
of  the  clay,  and  eventually  attained  a  considerable  thickness. 
Eventually,  however,  the  speed  of  the  current  once  more 


&f      f  ^':     '"*d 

^ 


DIAGONAL  BEDDING  IN  SANDSTONE,  MAOL  DONN,  ARRAN. 

From  H.M.  Geological  Survey's  Memoir,  "  The  Geology  of  North  Arran,  etc." 


[To  face  page 


STRATIFICATION  113 

increased,  sedimentation  was  again  locally  arrested,  and  the 
process  of  erosion  repeated,  the  sand  being  in  its  turn  trenched 
and  furrowed,  and  subsequently  covered  by  arenaceous  clay 


FIG.  8.— CONTEMPORANEOUS  EROSION. 

c,  t-i,  sandy  clay;  s,  si,  sandstone,  grit,  etc. 

(V1),  just  as  this  latter  became  denuded  and  afterwards  over- 
laid by  grit  and  sand  (j1). 

Grouping  of  Strata. — Although  almost  any  diversity  of 
strata  may  be  seen  in  one  and  the  same  vertical  section,  yet, 
as  might  have  been  expected,  it  is  usual  to  meet  with  rocks 
of  similar  character  associated  together.  Thus,  conglomerate 
is  more  frequently  interstratified  with  grit  and  sandstone,  than 
with  fine  argillaceous  deposits,  while  limestone  is  associated 
rather"  with  the  latter  than  with  coarser  grained  accumulations. 
Alternations  of  different  kinds  of  sediment  are  quite  char- 
acteristic of  the  deposits  now  forming  in  lakes,  estuaries,  etc., 
but  usually  the  passage  is  from  gravel  to  grit  and  sand,  and 
not  directly  from  shingle  and  gravel  to  silt  and  clay.  Even 
in  the  case  already  mentioned  (see  Fig.  5),  of  a  limestone 
intercalated  between  underlying  and  overlying  shales,  the 
change  from  the  one  to  the  other  is  not  always  so  abrupt  as 
it  may  seem  to  be.  Not  infrequently  it  will  be  found  that 
the  lower  shales  become  more  and  more  calcareous  towards 
the  top  of  the  stratum,  and  that  the  limestone,  in  like  manner, 
becomes  gradually  more  and  more  argillaceous  above — so 
that  there  is  a  sort  of  passage,  as  it  were,  from  the  one  kind 
of  rock  into  the  other. 

In  the  silting  up  of  lakes  and  estuaries,  however,  it  must 
happen  now  and  again  that  coarse  sediments  are  laid  down 

H 


114  STRUCTURAL  AND  FIELD  GEOLOGY 

directly  on  the  surface  of  fine  accumulations.  As  a  rapidly 
flowing  river  pushes  its  delta  outwards,  the  water  naturally 
shallows  in  front  of  the  advancing  alluvial  cone — in  other 
words,  the  zone  of  gravel  encroaches  upon  the  area  over  which 
sand  was  formerly  distributed,  while  the  sand  in  its  turn  is 
laid  down  upon  the  finer  mud  and  silt.  Conversely,  when 
sedimentation  takes  place  over  a  gradually  subsiding  area, 
finer  grained  deposits  continue  to  advance  shorewards  and 
extend  over  the  surface  of  coarser  accumulations.  In  general, 
however,  such  changes  are  only  developed  gradually,  so  that 
the  passage  from  one  kind  of  sediment  to  another  (either  in 
a  horizontal  or  a  vertical  direction),  will  not  usually  be  abrupt. 
But  in  the  case  both  of  an  advancing  delta  and  a  retreating 
coast-line,  sudden  changes  in  the  character  of  the  deposits 
must  occasionally  take  place.  During  floods  and  freshets,  for 
example,  the  coarser  detritus  hurried  forward  by  a  river  will 
make  an  abnormal  advance,  just  as  tidal  currents  will  now 
and  again  sweep  fine-grained  sediment  further  inshore  than 
usual.  Sudden  changes  like  this  are  often  accompanied  by 
the  process  already  described  as  "  contemporaneous  erosion." 
Contemporaneity  of  Strongly-contrasted  Strata.— When 
we  consider  that  sedimentary  deposits  are  in  process  of 
formation  over  enormous  stretches  of  sea-floor,  in  shallow 
and  deep  water  alike,  it  is  obvious  that  the  most  diverse 
accumulations  may  yet  be  of  contemporaneous  origin.  It  is 
no  more  than  we  might  expect,  therefore,  to  find  that  such 
rocks  as  grit  and  sandstone  have  been  formed  on  the  same 
sea-floor  as  limestone.  When  a  series  of  strata  is  traced 
across  a  wide  area,  we  constantly  see  some  of  the  beds  thin- 
ning off,  and  their  position  in  the  sequence  being  occupied  by 
others  of  a  different  kind.  In  this  way  a  great  succession  of 
thick-bedded  limestones  may  be  split  up,  as  it  were,  by  the 
intercalation  of  shales  and  sandstones,  which  continuously 
increase  in  thickness,  while  the  limestones  at  the  same  time 
gradually  get  thinner  and  thinner  until  at  last  they  disappear, 
and  the  whole  series  of  strata  then  comes  to  consist  of  sand- 
stones and  shales  alone.  Similar  changes  are  brought  about 
by  variations  in  the  character  of  the  individual  beds  them- 
selves. Conglomerate,  as  we  have  seen,  gradually  shades  off 
into  pebbly  grit  and  sandstone,  just  as  siliceous  sandstones 


STRATIFICATION  1 1 5 

pass  laterally  into  fine-grained  argillaceous  sandstones,  and 
these  in  their  turn  eventually  merge  into  shales.  So  lime- 
stone tends  to  become  mixed  with  clay  or  sand,  and  to  shade 
off  into  calcareous  shales  or  sandstones.  Again,  coal  and 
ironstone  may  mutually  replace  each  other ;  or  each  may 
lose  its  own  distinctive  character  and  gradually  pass  into 
carbonaceous  or  ferruginous  shale. 

An  example  of  a  well-marked  group  of  strata  which  gradually  changes 
its  character  as  it  extends  from  one  region  to  another  is  supplied  by 
the  Lower  Oolite  of  England.  This  formation  may  be  followed  from 
Somerset  through  Gloucester  and  the  Midlands  to  the  Humber. 
Throughout  its  whole  course  it  rests  upon  and  is  covered  by  well-defined 
argillaceous  beds — the  Lias  below  and  the  Oxford  Clay  above.  In  the 
south  of  England  the  formation  is  composed  essentially  of  limestones. 
Followed  to  the  north,  however,  it  becomes  more  and  more  arenaceous 
and  argillaceous,  until  in  Yorkshire  the  limestones  of  the  southern  district 
are  entirely  replaced  and  represented  by  ordinary  sandstones  and  shales 
with  associated  coals  and  ironstones.  The  transformation  of  the  deposits 
is  not  hard  to  understand.  The  calcareous  accumulations  of  the  south 
are  obviously  marine,  while  the  arenaceous  and  argillaceous  deposits  of 
the  north  are  of  estuarine  and  brackish  water  origin. 

A  somewhat  similar  change  comes  over  the  great  Carboniferous 
Limestone  formation  when  it  is  followed  from  England  into  Scotland. 
In  the  Mendip  Hills  the  formation  consists  almost  entirely  of  limestones, 
which  reach  a  thickness  of  3500  feet  at  least.  In  Northumberland,  the 
limestone  series  of  the  south  is  represented  by  a  great  succession  of 
sandstones  and  shales,  with  associated  coal-seams  and  beds  of  limestone 
that  vary  individually  in  thickness  from  7  feet  to  150  feet — the  entire 
formation  ranging  from  2500  feet  to  upwards  of  6000  feet.  In  Scotland 
the  arenaceous  and  argillaceous  element  acquires  a  very  great  develop- 
ment— probably  not  less  than  10,000  feet.  The  only  limestones  present, 
however,  are  some  half-dozen  beds,  varying  in  thickness  from  a  few  feet 
up  to  20  or  30  yards,  which,  along  with  numerous  seams  of  coal  and 
ironstone,  are  intercalated  in  the  upper  part  of  the  series. 

Diagonal  (Oblique  or  Cross-)  Lamination  and  Strati- 
fication.— While  it  is  generally  true  that  sedimentary 
deposits  are  spread  out  in  approximately  horizontal  sheets, 
now  and  again  both  laminae  and  bedding  show  much  irregu- 
larity— not  only  the  individual  beds,  but  the  layers  of  which 
they  are  composed,  being  often  inclined  to  each  other  at 
various  angles.  The  structure  is  shown  in  Plate  XXIV.,  and 
owes  its  origin  to  changes  or  oscillations  in  the  direction  and 
force  of  currents.  Hence  it  is  often  termed  "  current-bedding  " 
or  "  false-bedding,"  in  reference  to  the  fact  that  the  bedding 


116  STRUCTURAL  AND  FIELD  GEOLOGY 

does  not  indicate  that  of  the  series  of  strata  in  which  it  occurs. 
The  structure  is  common  in  littoral  deposits  and  accumula- 
tions formed  in  shallow  water,  where  there  is  much  shifting 
and  eddying  of  current-action.  Not  infrequently  a  highly 
false-bedded  sandstone  is  directly  underlaid  and  overlaid  by 
evenly  bedded  strata,  which  give  evidence  of  quiet  and 
undisturbed  sedimentation. 

False-bedding  of  a  pronounced  character  is  characteristic  of  the 
deltas  formed  by  torrential  streams  and  rivers.  Such  deltas  advance 
more  or  less  rapidly,  and  usually  present  a  somewhat  steeply  sloping 
front.  As  already  explained,  gravel  and  coarse  detritus  are  shot 
forward  by  the  current,  and  roll  down  this  steep  bank — the  finer  sediment 
being  carried  further  and  coming  to  rest  on  the  bed  of  the  lake  or 
estuary,  so  as  to  form  approximately  horizontal  accumulations.  Thus  in 
time,  as  the  bank  advances,  steeply-inclined  beds  of  gravel  (g)  come  to 
overlie  horizontal  sheets  of  sand  (s]  and  silt  (;«),  (Fig.  9). 


FIG.  9. — DELTA  FORMED  BY  TORRENTIAL  STREAM. 

g,  gravel ;  t>,  sand ;  m,  silt. 

Surface-Markings. — The  surfaces  of  derivative  rocks 
often  exhibit  interesting  markings,  among  which  the  most 
notable  are  current-marks.  These  are  of  precisely  the  same 
character  as  the  ripple-marks  seen  on  modern  sea-beaches 
(see  Plate  XXV.  2).  In  the  shallow  water  of  a  sea-beach  the 
ripple-marks  slowly  advance  with  the  inflowing  tide.  They 
usually  present  a  long,  gentle  slope  seawards,  and  a  short  and 
more  abrupt  slope  towards  the  shore.  With  the  ebb-tide,  the 
crests  of  the  ridges  tend  to  be  smoothed  off  or  truncated. 
When  the  movement  of  the  water  is  irregular,  as  between 
skerries,  boulders,  etc.,  the  result  is  the  formation  of  numerous 
miniature  hummocks  and  dimples,  or  straggling  hollows  and 
rounded  ridges. 

Although  current -marks  are  most  commonly  associated  with  beach- 
deposits,  they  are  not  confined  to  these,  but  may  originate  at  any  depth  io 
which  the  agitation  of  the  water  extends.  They  have  been  observed  forming 
in  clear  water  at  depths  of  50  feet  and  more.  Ripple-marked  surfaces  are 
common  in  many  sandstones  and  argillaceous  beds,  and  often  occur  one 
over  another  throughout  a  thick  series  of  strata.  As  each  advancing  tide 


i.  RILL-MARKS  ON  BEACH  AT  ELIE,  FIFE. 

Photo  by  Dr  Laurie. 


CURRENT-MARKS  IN  CARBONIFEROUS  SANDSTONE,  SHORE  NEAR  ST  MONAN'S,  FIFE. 

Photo  by  Dr  Laurie. 

[To  face  page  116. 


STRATIFICATION  117 

effaces  old  marks,  and  replaces  these  by  new  ones,  it  is  difficult  to 
understand  h6w,  under  ordinary  conditions,  a  rippled  surface  of  sand  can 
be  preserved.  Hence  it  has  been  surmised  that  many  of  the  ripple- 
marked  surfaces  which  appear  in  rocks  of  all  geological  ages  may 
have  been  produced  below  low  tide-level,  in  shallow  bays  or  in  estuaries, 
where  sedimentation  is  more  or  less  continuously  carried  on  ;  so  that 
rippled  surfaces  might  be  often  preserved  by  the  gentle  deposition  upon 
them  of  fresh  accumulations  of  sediment.  However  that  may  be,  it 
seems  certain  that  not  infrequently  ripple-marked  surfaces  have  really 
been  formed  between  high  and  low  water.  In  some  cases  these  have 
probably  been  preserved  by  the  deposition  over  their  surface  of  a  thin 
film  of  clay.  In  other  cases  the  rippled  sand  (often  to  some  extent 
argillaceous)  had  become  sufficiently  consolidated  to  resist  the  action  of 
the  next  incoming  tide.  It  must  be  remembered,  that  at  low  tide  on 
gently  shelving  shores  a  wide  expanse  of  beach  is  laid  bare.  Exposed 
to  the  rays  of  a  hot  sun,  the  fine-grained  sand  or  sandy  mud  might  thus, 
over  wide  areas,  be  so  dried  and  hardened  as  to  resist  the  obliterating 
action  of  the  flowing  tide,  and  under  such  circumstances  it  is  conceivable 
that  surface  after  surface  might  be  covered  up  and  preserved.  Again, 
on  flat  shores,  wide  belts  of  rippled  sand  and  mud  might  be  exposed 
between  the  lines  of  spring  and  neap  tides.  Hence,  the  surfaces  above 
high-water  of  ordinary  tides  might  become  dried  and  consolidated  before 
thev  were  eventually  covered  by  newer  accumulations.  The  layer 
irrMediately  overlying  a  ripple-marked  surface  usually  shows  a  more  or 
le^s  perfect  cast,  which,  when  removed  from  its  position,  is  often  hard  to 
wi€tinguish  from  the  actual  mould  or  original  surface.  Frequently, 
however,  the  hollows  are  more  sharply  pronounced  than  the  ridges,  and 
wheJJkuch  is  the  case  the  cast  of  a  hollow  would  show  a  sharp  crest, 
not  be  formed  on  the  summit  of  a  ridge. 


Wave-marks.  —  These  are  seen  forming  on  modern  sea- 
beaches  during  ebb-tide.  They  are  delicate  outlinings 
which  mark  the  limits  reached  by  the  waves  as  they  die 
out.  If  the  edge  of  the  thin  layer  of  advancing  water  be 
observed,  it  will  be  seen  that  it  sweeps  along  with  it  fine 
grains  of  sand,  and  more  particularly  particles  which,  by 
reason  of  their  shape  (mica-flakes)  or  light  specific  gravity, 
(coaly  matter)  are  readily  carried  forward.  When  the  wavelet 
dies  out,  these  materials  are  stranded  so  as  to  form  a 
miniature  ridge,  which  is  often  rendered  conspicuous  by  the 
presence  of  black  carbonaceous  matter.  Wave-marks  of  this 
kind  are  not  infrequently  seen  on  the  surfaces  of  fine-grained 
sandstones  and  flagstones,  and  are  good  evidence  of  a  beach- 
formation. 

Rill-marks  (see  Plate  XXV.  i).  —  These  are  small  furrows 


118  STRUCTURAL  AND  FIELD  GEOLOGY 

formed  on  a  sandy  or  muddy  beach  by  the  trickling  down- 
wards of  little  rills  during  the  retreat  of  the  tide.'  They  are 
occasionally  visible  on  the  surfaces  of  fine-grained  sedimentary 
rocks.  When  they  are  numerous  and  run  into  each  other, 
they  often  simulate  the  appearance  of  some  kind  of  algae,  and 
have  not  infrequently  been  described  as  fossil  sea-weeds. 

Sun-cracks  (Plate  XXVI.). — Round  the  shores  of  inland 
seas  and  lakes,  the  level  of  which  is  liable  to  fall  during  the 
dry  season  of  the  year,  a  wide  belt  of  gently  shelving  ground 
is  laid  bare.  The  same  is  the  case  in  many  river-valleys — 
broad  flats  appearing  when  the  rivers  are  low.  Frequently 
such  exposed  tracts  consist  of  clay  or  mud,  which,  under  the 
influence  of  the  sun,  becomes  dry  and  shrinks,  so  that  the 
surface  cracks  into  polygonal  cakes.  When  the  wet  season 
arrives,  and  the  level  of  the  water  rises,  sand  may  be  deposited 
over  the  consolidated  and  fissured  clay,  and  thus  a  cast  of  the 
cracks  will  be  formed.  The  same  action  may  take  place  on 
low,  flat  beaches  which  are  exposed  to  a  hot  sun  during  the 
retreat  of  the  tide.  Sun-cracks  are  thus  of  common  occurrence 
in  many  geological  systems.  The  casts  usually  adhere  to  $he 
overlying  stratum,  of  which  indeed  they  form  a  part. 

Rain-prints. — In  like  manner  the  pits  made  by  rain  on  the 
surface  of  fine-grained  deposits  have  occasionally  beerj.  ^re- 
served. The  smooth  bedding-planes  of  argillaceous/,/^  id- 
stones,  shales,  and  mudstones  are  not  infrequently  pinfcd  in 
this  way,  the  casts  of  the  pits  occurring  on  the  under  surface 
of  the  overlying  stratum.  Sometimes  the  direction  of  the 
wind  at  the  time  the  rain  fell  is  shown  by  the  inclination  of 
the  pits  in  one  particular  direction.  In  such  cases  there  is 
occasionally  the  appearance  of  a  slight  ridge  on  one  side  of 
the  pits,  as  if  some  of  the  fine  sediment  had  been  flicked  out 
by  the  drops  as  they  fell. 

Animal-tracks,  etc. — Additional  evidence  of  beach-con- 
ditions is  obtained  from  tracks  left  by  animals.  Thus,  tracks 
or  trails  of  annelids,  molluscs,  crustaceans,  etc.,  worm-burrows 
and  castings,  and  the  footprints  of  reptiles,  amphibians,  birds, 
and  mammals,  have  been  preserved,  usually  in  fine-grained 
sedimentary  strata.  Now  and  again,  also,  certain  puzzling 
impressions  make  their  appearance,  which  have  often  been 
described  as  plant-marks.  Possibly  some  of  these  may  have 


-vl 

\ 


•AST  OF  SUN-CRACKS  IN  SANDSTONE.     One-third  natural  size. 


2.  ANOTHER  SPECIMEN.     About  half  natural  size. 

[To  face  page  118. 


STRATIFICATION  119 

been  formed  by  sea-weeds  waved  to  and  fro  by  eddying  waters, 
the  fronds  of  the  algae  brushing  the  surface  of  the  sand  or 
mud,  and  thus  drawing  or  etching  curved  patterns.  Others, 
again,  may  represent  the  trails  made  by  floating  algae  or  by 
the  tentacles  of  a  jelly-fish.  Various  surface-markings  which 
mimic  organic  structures  and  have  been  given  generic  and 
even  specific  names  are  probably  often  of  mechanical  origin, 
formed  either  during  or  after  consolidation  of  the  rocks  in 
which  they  occur. 


CHAPTER  VIII 

CONCRETIONARY  AND   SECRETIONARY  STRUCTURES 

Siliceous  Concretions — Flint,  Chert,  Menilite.  Calcareous  and  Ferrugin- 
ous Concretions — Septaria,  Composite  Nodules,  Rattle-stones,  Fairy- 
stones,  Kankar,  etc.  Clay-ironstone  Nodules,  Pyrite,  Marcasite, 
Gypsum,  Dendrites.  Concretionary  Sandstones,  Argillaceous  Rocks, 
and  Limestones.  Concretionary  Tuffs.  Concretions  in  Crystalline 
Igneous  Rocks.  Secretionary  Structures  —  Amygdules,  Geodes, 
Drusy  Cavities. 

Concretionary  Structures  may  occur  in  almost  any  kind 
of  derivative  rock.  Sometimes  they  affect  the  mass  of  a  rock  ; 
at  other  times  they  take  the  form  of  various  sized  spherical  or 
lenticular  bodies  or  nodules,  scattered  regularly  or  irregularly 
through  a  rock,  or  they  may  appear  as  more  or  less  interrupted 
layers  or  vertical  and  ramifying  veins,  or  as  well-formed 
crystals.  In  most  cases  they  owe  their  origin  to  the  gradual 
aggregation  of  mineral  matter  originally  diffused  through  the 
mass  of  the  rock  in  which  they  occur.  Occasionally,  however, 
the  mineral  matter  has  been  introduced  from  the  outside  by 
percolating  water.  The  commonest  concretions  are  siliceous, 
calcareous,  and  ferruginous,  and  there  is  a  strong  tendency 
in  spherical  concretions  to  assume  internal  radiating  and 
concentric  structures. 

(a)  SILICEOUS. — Among  the  most  familiar  examples  of  siliceous  con- 
cretions are  the  _/&'«/.$•,  which  occur  so  frequently  in  chalk.  Flint  nodules 
'  are  usually  irregular  in  form,  and  vary  in  size  up  to  a  foot  or  more  in 
diameter.  They  are  white  externally,  and  brown  to  black  internally. 
They  often  enclose  or  partially  enclose  fossils,  more  particularly  sponges. 
Usually  they  are  arranged  in  lines  that  coincide  with  the  bedding-planes 
of  the  chalk.  They  may  coalesce  to  form  more  or  less  interrupted  sheets 
or  seams  of  flint  (some  three  or  four  inches  thick),  which  follow  the  bed- 
ding of  the  chalk,  or  may  traverse  it,  as  irregular  vertical  or  ramifying 
veins.  In  the  older  limestones  chert  plays  much  the  same  part  as  flint 

120 


i.  SECTION  OF  SEPTARIAN  NODULE  (CLAY-IRONSTONE).    About  one-half  natural  siz 


2.  SECTION  OF  SPHERICAL  CONCRETIONS  (FERRUGINOUS)  IN  SANDSTONE. 
About  one-half  natural  size. 


PLATE  XXVIII. 


DENDRITIC  MARKINGS  (PSILOMELANE)  ON  LIMESTONE.    Two-thirds  natural  size. 


[Between  pages  120  and  121 


CONCRETIONS  AND  SECRETIONS  121 

in  chalk,  and  is  probably,  like  the  latter,  in  many  cases  of  organic  origin. 
Now  and  again,  however,  it  may  have  been  a  deposition  from  thermal 
water.  In  such  cases  it  occurs  as  thin  laminae,  interleaved  with  similar 
laminae  of  limestone— the  layers  being  often  highly  puckered,  crumpled,  or 
confusedly  contorted  and  involved,  as  if  the  deposits  had  been  disturbed  by 
the  bubbling  up  of  spring-water  before  they  had  become  quite  solidified. 
These  appearances,  however,  may  be  otherwise  accounted  for.  The 
siliceous  solution  may  have  been  originally  in  a  colloid  or  jelly-like 
condition,  containing  some  percentage  of  water.  Thus,  when  the  mineral 
began  to  lose  its  water  and  solidify,  the  contraction  of  its  bulk  would  give 
rise  to  much  distortion  and  confusion — later  accretions  of  silica  filling  up 
any  fissures  or  cavities  thus  produced.  Siliceous  secretions  are  not 
uncommon  in  some  argillaceous  rocks  :  nodules  and  seams  of  chert 
(often  radiolarian),  for  example,  occasionally  occur  in  Palaeozoic  shales, 
and  reniform  menilite  appears  now  and  again  in  marls  of  later  age. 
Even  sharp  crystals  of  quartz^  generally  of  small  size,  have  occasionally 
been  developed  in  marly  clay. 

(<£)  CALCAREOUS  and  FERRUGINOUS. — Spherical  and  nodular  cal- 
careous and  ferruginous  concretions  are  characteristic  of  many  argillaceous 
rocks  and  of  some  sandstones.  In  laminated  clay  the  mineral  solutions 
have  made  their  way  most  readily  along  the  planes  of  sedimentation,  so 
that  the  resulting  concretions  are  usually  somewhat  lenticular,  and  often 
assume  the  shape  of  flattened  spheroids.  When  numerous,  they  not 
infrequently  coalesce  so  as  to  form  irregular  concretionary  bands  or 
layers.  Very  often,  however,  they  are  scattered  sporadically  through  the 
beds  in  which  they  occur.  In  homogeneous  clay-rocks,  without  apparent 
lamination,  the  concretions  are  usually  either  spherical  or  variously 
shaped,  and  often  irregularly  dispersed.  Frequently  they  have  formed 
round  a  nucleus,  which  may  consist  of  mineral  matter,  but  is  more 
commonly  of  organic  nature,  such  as  a  shell,  a  coprolite,  a  fish,  a 
fragment  of  plant,  etc.  A  concretion  may  be  compact  and  homogeneous 
throughout,  or  may  consist  of  concentric  shells,  or  while  externally 
compact  it  may  be  much  cracked  and  fissured  internally.  The  cracks 
are  widest  towards  the  centre  of  a  concretion,  and  die  out  towards  its 
circumference,  as  if  the  interior  had  contracted  after  the  outside  had  dried 
and  become  consolidated.  They  are  often  partially  or  completely  filled 
with  subsequently  introduced  mineral  matter,  usually  calcite.  Concretions 
of  this  kind  are  known  as  septaria  or  septarian  nodules,  in  allusion  to  the 
septation  or  partitioning  of  the  interior  (see  Plate  XXVII.  i).  Septaria 
are  commonly  either  calcareous  or  ferruginous.  Occasionally,  concretions 
consist  of  concentric  shells  of  different  chemical  composition.  In  a 
nodule,  composed  for  the  most  part  of  ferruginous  matter,  one  or  more 
of  the  shells  may  be  calcareous  ;  or  the  core  or  kernel  may  be  calca- 
reous, and  the  external  shells  ferruginous.*  Owing  to  the  subsequent 
action  of  percolating  water,  the  calcareous  portions  may  be  completely 

*  Occasionally  one  or  more  of  the  concentric  shells  may  consist  of 
oxide  of  manganese  (psilomelane). 


122  STRUCTURAL  AND  FIELD  GEOLOGY 

removed  in  solution.  In  this  way,  by  the  removal  of  a  calcareous  layer 
from  the  interior  of  a  nodule,  a  central  ferruginous  kernel  becomes 
detached,  and  rattles  when  the  concretion  is  shaken  (Klapperstein,  or 
Rattle-stone).  When  a  calcareous  core  is  entirely  dissolved,  a  nodule,  of 
course,  becomes  hollow.  Many  nodules,  however,  are  rendered  hollow, 
simply  owing  to  the  contraction  of  the  interior  after  the  outer  shell  has 
dried  and  hardened. 

Concretions  of  the  several  kinds  referred  to  in  the  preceding  paragraph 
are  all  obviously  of  secondary  origin — they  are  superinduced  structures. 
This  is  shown  by  the  fact  that  the  planes  of  sedimentation  can  often  be 
seen  passing  through  them,  and  never  curving  over  them,  as  would  have 
been  the  case  had  they  been  loose  stones  and  boulders  covered  up 
while  lying  on  lake-floor  or  sea-bottom.  Among  familiar  examples  of 
concretionary  calcareous  nodules  are  the  so-called  fairy-stones  so  fre- 
quently met  with  in  alluvial  clays.  In  Germany  they  are  common  in 
loess,  and  are  known  to  the  country-folk  as  "  Loss-piippchen,  Loss- 
mannchen,"  etc.,  and  as  "  Marlekor"  (Kobolds'  playthings)  or  "  Nakkebrod" 
(Nixies'  bread)  in  Sweden.  Similar  calcareous  concretionary  nodules 
termed  "kankar"  are  abundantly  developed  in  many  of  the  alluvial 
deposits  of  India.  Reference  may  also  be  made  to  the  curious  calcareous 
concretionary  structures  which  occur  in  the  Tertiary  sand  of  Fontainebleau, 
near  Paris.  These  frequently  take  the  form  of  single  crystals  of  calcite, 
or  of  groups  and  aggregates  of  such  rhombohedral  crystals.  Ferruginous 
concretions  are  well  represented  in  this  country  by  the  balls  and 
nodules  of  clay -ironstone  (sphaerosiderite)  that  occur  so  abundantly  in  the 
black  shales  of  the  Carboniferous  System.  They  vary  in  size  from  a  hazel- 
nut  to  flattened  spheroids  measuring  two  or  three  feet  across,  but  these  last 
are  not  common.  Many  contain  a  fossil  at  the  centre,  while  others  seem 
to  consist  wholly  of  inorganic  materials.  A  large  proportion,  it  may  be 
added,  are  septarian.  As  calcareous  and  ferruginous  concretions  alike 
tend  to  be  developed  in  the  direction  of  the  bedding-planes  of  the  rock 
in  which  they  occur,  it  frequently  happens  that  contiguous  nodules  become 
fused  together,  so  as  to  form  more  or  less  continuous  seams  of  limestone 
or  of  ironstone,  as  the  case  may  be.  Sometimes  such  seams  maintain  an 
uniform  thickness,  but  more  usually  they  are  lumpy,  thickening  and 
thinning  irregularly.  Concretions  of  disulphide  of  iron  (Pyrite  and 
Marcasite)  are  of  frequent  occurrence  in  sandstone,  clay,  chalk,  and  coal. 
They  vary  in  size  from  minute  grains  up  to  nodules  two  or  three  inches  in 
diameter.  In  the  form  of  nodular  concretions  marcasite  is  much  more 
common  than  pyrite,  the  concretions  having  usually  an  internal,  fibrous, 
radiating  structure.  Now  and  again  marcasite,  however,  assumes  a 
crystalline  form,  as  in  the  flat,  spear-headed  "  twins  "  which  are  seen  in 
the  chalk  deposits  at  Dover  and  Folkestone.  Pyrite  does  not  occur  so 
commonly  in  nodules  as  marcasite,  but  has  a  much  wider  distribution  in 
the  crystalline  form,  crystals  and  crystalline  aggregates  appearing  in 
many  kinds  of  derivative  rocks,  either  dispersed  through  a  rock-mass 
or  lining  its  minute  cracks  and  fissures.  As  sporadic  crystals  or  groups 
of  crystals,  it  often  appears  in  clay-slate,  but  such  occurrences  fall  to  be 


CONCRETIONS  AND  SECRETIONS  123 

considered  under  the  head  of  metamorphism.  No  hard-and-fast  line  can 
be  drawn  between  the  changes  which  produce  concretions  and  concre- 
tionary structures  in  "unaltered"  rocks,  and  those  which  have  induced 
the  aggregation  and  crystallisation  of  mineral  matter  in  certain  "  altered  " 
or  "  metamorphic  "  rocks. 

Sulphate  of  lime  is  not  so  often  met  with  in  concretions  as  carbonate 
of  lime  and  ferruginous  compounds.  In  some  clay-rocks,  however, 
gypsum  concretions  are  common  enough.  Sometimes  these  appear  as 
large  perfect  crystals  and  twins  of  the  mineral,  but  more  frequently  as 
lenticular  nodules,  or  layers,  an  inch  or  more  in  thickness. 

The  oxides  of  manganese  and  iron  occur  not  only  in  nodular  forms, 
but  frequently  appear  as  thin  films  coating  the  surfaces  of  the  natural 
division-planes  of  rocks,  such  as  joints  and  bedding-planes.  They  usually 
assume  delicate  plumose  or  plant-like  forms  resembling  sprigs  of  moss, 
etc.,  and  hence  are  termed  dendrites  or  dendritic  markings  (see  Plate 
XXVIII.).  Although  usually  appearing  only  on  division-planes,  now 
and  again  they  ramify  through  the  substance  of  fine-grained  rocks,  such 
as  certain  limestones,  on  sections  of  which  the  markings  often  simulate 
belts  of  trees,  hedgerows,  etc.  (landscape-marble}. 

CONCRETIONARY  ROCKS.— Not  only  do  mineral  solutions  tend  to 
form  concretions  of  various  kinds  in  rocks,  but  the  rocks  themselves  have 
not  infrequently  acquired  a  concretionary  structure.  Some  sandstones,  for 
example,  seem  to  be  largely  composed  of  aggregates  of  ball-like  or  larger 
spheroidal  masses.  Few  sandstones,  indeed,  do  not  in  places  show  some 
indications  of  this  concretionary  structure.  The  spheroids  are  now  and 
again  enclosed  in  dark  brown  ferruginous  crusts,  the  rock  within  being 
often  bleached,  and  it  may  even  be  reduced  to  the  condition  of  loose 
sand.  When  sandstone  of  this  character  is  exposed  by  quarrying,  the 
freshly  cut  rock  may  show  concentric  bands  of  a  dark  brown  or  red 
colour,  some  of  which  may  be  an  inch  or  less  in  width,  while  others  may 
exceed  several  feet.  The  origin  of  the  structure  is  obscure.  The 
ferruginous  matter  may  have  been  introduced  by  percolating  water,  but 
some  of  it  at  least  has  been  abstracted  from  the  sandstone  itself.  The 
concentric  shells  of  ferruginous  matter  shown  in  Plate  XXVII.  2  are  not 
hard  to  explain,  and  their  mode  of  formation  may  throw  some  light  on 
that  of  the  larger  concretionary  masses  to  which  reference  has  just  been 
made.  They  owe  their  origin  undoubtedly  to  the  presence  of  disseminated 
granules  or  crystals  of  some  ferruginous  mineral,  almost  certainly  pyrite 
or  marcasite.  By  the  action  of  water  soaking  into  the  stone  the  mineral 
is  broken  up  chemically,  and  a  ferruginous  solution  formed,  which  spreads 
outwards  as  a  drop  of  ink  does  on  blotting-paper.  Evaporation  taking 
place  around  the  outer  margin  of  the  solution,  iron-oxide  is  precipitated,  and 
the  first  ring  or  shell  is  formed.  The  process  is  repeated  by  the  formation 
of  a  second  shell  inside  the  first,  and  thereafter  the  production  of  successive 
concentric  shells  is  continued,  each  forming  inside  of  its  predecessor, 
until  the  ferruginous  solution  is  exhausted.  In  some  cases,  a  portion  of 
the  ferruginous  mineral  at  the  centre  may  remain,  but  it  is  usually  so 
small  and  so  much  altered  that  its  original  character  is  hardly  recognisable. 


124  STRUCTURAL  AND  FIELD  GEOLOGY 

Other  kinds  of  concretionary  structures  are  frequently  met  with  in  sand- 
stones. Small  quantities  of  carbonate  of  lime  or  carbonate  of  iron, 
diffused  through  the  rock,  tend  to  aggregate  so  as  to  form  irregular 
concretionary  masses  of  sandstone,  which  are  much  harder  than  the 
surrounding  rock.  Cracks  and  crevices  in  concretionary  masses  of  this 
kind  are  often  filled  or  lined  with  crystalline  siderite  or  calcite,  as  the  case 
may  be.  The  hardened  rock  (known  as  "kingle"  in  Scotland)  breaks 
with  a  splintery  fracture,  and  is  rejected  by  the  quarrymen  as  unsuitable 
for  building  purposes. 

Argillaceous  rocks  hardly  less  frequently  assume  concretionary  forms. 
Now  and  again  a  whole  bed  of  shale  may  exhibit  the  structure — the 
rock  appearing  to  be  composed  of  an  aggregate  of  various  sized 
spheroids.  The  spheroids  usually  show  a  concentric  arrangement — the 
concentric  shells  being  in  some  cases  separated  from  each  other  by  thin 
films  of  ferruginous  matter. 

Calcareous  rocks  often  enough  acquire  a  concretionary  structure — 
the  most  pronounced  examples  of  the  kind  being  furnished  by  dolomitic 
or  magnesian  limestone,  as  already  described  (p.  65).  Reference  may 
also  be  made  to  the  oolitic  structure  of  certain  limestones,  calcareous 
tufas,  and  ironstones,  which,  however,  in  most  cases  is  original  (see  p.  70). 

Concretionary  structures,  comparable  to  those  that  characterise  so 
many  derivative  rocks,  can  hardly  be  said  to  occur  in  igneous  rocks. 
Exception,  however,  must  be  made  of  the  tuffs,  in  many  of  which  concre- 
tionary ferruginous  and  calcareous  nodules  occur,  while  now  and  again 
tuff  itself  may  exhibit  concretionary  structure,  such  as  that  seen  occasion- 
ally in  argillaceous  shales.  But  the  concretionary  structures  that  affect 
many  crystalline  igneous  rocks  differ  from  those  which  occur  in  derivative 
rocks  in  being  original  and  not  superinduced.  For  example,  the  dark, 
irregular  shaped  aggregates  of  ferromagnesian  minerals  which  appear  in 
many  granites,  gabbros,  and  other  plutonic  masses,  and  the  sporadic 
nodular  masses  of  olivine  so  frequently  met  with  in  basalt,  are  early 
segregations  from  the  original  molten  magma.  They  are  not,  like  the 
septarian  nodules  described  above,  younger  than  the  rocks  in  which  they 
occur.  So,  again,  the  aggregates  of  spherical  bodies  which  constitute 
orbicular  diorite  or  napoleonite  are  not  superinduced  but  original 
structures — they  consist  of  radially  and  concentrically  arranged  felspar 
and  hornblende — the  two  primary  and  essential  ingredients  of  the  rock 
(see  Plate  XIV.).  Similar  original  concretionary  structures  are  met  with 
in  other  crystalline  igneous  rocks,  as,  for  example,  the  ball-granite 
("  Kugel-granit ")  of  Finland. 

Secretionary  Structures. — These  are  especially  character- 
istic of  certain  types  of  igneous  rocks,  but  may  occur  in 
almost  any  kind  of  rock  having  a  cellular  or  cavernous 
structure.  They  consist  of  mineral  matter  which  has  been 
deposited  on  the  walls  of  cavities,  usually  in  successive 
layers,  and  thus  they  may  be  said  to  increase  from  without 


CONCRETIONS  AND  SECRETIONS  125 

inwards.  In  this  respect  they  differ  from  concretions  which 
owe  their  origin,  as  already  explained,  to  the  aggregation  of 
mineral  matter  round  a  central  point,  so  that  they  grow  from 
within  outwards.  Secretions  are  typically  represented  by  the 
mineral  matter  which  so  often  occupies  the  vapour  cavities 
of  ancient  lava-form  rocks.  As  these  cavities  are  usually 
somewhat  flattened  from  having  been  drawn  out  in  the 
direction  of  flow,  the  subsequently  introduced  secretions  are 
often  almond-shaped.  Hence  they  are  termed  amygdtdes, 
and  the  rock  itself  is  said  to  be  amygdaloidal.  Such  cavities 
vary  in  size  from  mere  pores  up  to  hollows  measuring  many 
inches  in  diameter.  Sometimes  the  walls  are  lined  with  a 
mere  film  of  mineral  matter ;  in  other  cases  the  cavities  may 
be  largely  or  completely  filled  up  (see  Plate  I.  i).  A  secretion 
may  consist  of  one  and  the  same  kind  of  mineral  matter,  or 
of  successive  bands  of  various  minerals,  and  some  of  these 
bands  may  be  distinctly  crystalline,  while  others  are  crypto- 
crystalline  or  apparently  amorphous.  In  other  cases  a  cavity 
may  be  occupied  by  an  irregular  aggregate  of  different 
minerals — all  more  or  less  well  crystallised.  A  hollow 
secretion,  readily  separable  as  a  nodule  from  the  rock  in 
which  it  was  formed,  is  termed  a  geode ;  while  druse  is  the 
term  applied  to  a  cavity  which  is  lined  or  studded  with 
crystals.  Nevertheless,  "  geode  "  and  "  druse  "  are  sometimes 
used  interchangeably.  It  is  common,  for  example,  to  apply 
the  term  geode  to  siliceous  secretions  occurring  in  the  form 
of  hollow  spheroids  or  balls,  in  such  rocks  as  limestone  and 
highly  decomposed  amygdaloids,  from  which  the  ball-like 
bodies  are  readily  detached.  "Geode,"  therefore,  refers 
rather  to  the  secretion  than  to  the  cavity  in  which  it  occurs. 
"  Druse,"  on  the  other  hand,  has  reference  not  only  to  a  par- 
ticular character  of  the  secretion,  but  to  the  fact  that  it  occu- 
pies a  cavity.  Hence,  geologists  often  speak  of  drusy  cavities, 
meaning  by  that  simply  crystal-lined  hollow  spaces. 

The  secretions  occurring  in  crystalline  igneous  rocks  may 
be  (a)  original  or  synchronous,  or  (b]  subsequent  or  superinduced. 
As  types  of  the  former  (original}^  may  be  cited  the  drusy 
cavities  in  granite  (Plate  X.  2),  which  are  partially  filled  with 
well-crystallised  examples  of  one  or  more  of  the  original 
constituents  of  the  rock.  Obviously,  such  secretions  must 


126  STRUCTURAL  AND  FIELD  GEOLOGY 

be  synchronous  with  the  formation  of  the  granite.  Analogous 
to  the  drusy  cavities  in  granite  are  the  mineral-lined  or 
mineral-filled  cavities  of  irregular  shape  which  are  char- 
acteristic of  some  acid  igneous  rocks  (rhyolite).  These,  it 
can  hardly  be  doubted,  are  deposits  from  heated  solutions, 
formed  before  the  rock  in  which  they  occur  had  cooled.* 
Probably  of  similar  origin  (at  least  in  some  cases)  are  the 
zeolites,  which  occur  so  abundantly  in  the  vapour  cavities  of 
certain  basic  rocks,  as,  for  example,  the  fine  drusy  cavities 
of  the  Tertiary  basalts  of  the  Faeroe  Islands  and  Iceland. 

Among  subsequent  secretions,  the  most  typical  are  the 
amygdules  referred  to  above.  While  the  formation  of  these 
may  sometimes  be  almost  synchronous  with  that  of  the  rock 
in  which  they  occur,  there  can  be  little  doubt  that  in  most 
cases  the  amygdules  are  of  subsequent  origin,  the  mineral 
matter  having  been  introduced  by  percolating  water  long  after 
the  cellular  rock  had  cooled  and  solidified.  Amygdaloidal 
rocks  are  usually  more  or  less  decomposed,  the  amygdules 
consisting  of  material  derived  from  the  breaking  up  of  one 
or  more  of  the  original  rock-constituents,  especially  the 
felspars. 

The  minerals  of  most  frequent  occurrence  in  amygdaloidal 
cavities  are  calcite,  chalcedony  (agates),  quartz,  zeolites, 
green-earth,  etc.  As  siliceous  secretions  are  more  durable 
than  the  igneous  rocks  in  which  they  occur,  they  are  often 
found  in  the  soils  and  subsoils  resulting  from  the  decom- 
position of  amygdaloidal  rock,  and  under  such  conditions 
they  are,  as  already  indicated,  often  termed  geodes. 

*  There  can  be  little  doubt  that  the  mineral  composition  of  igneous 
rocks  has  sometimes  been  greatly  affected  by  such  heated  solutions. 
For  example,  the  kaolin  or  china-clay  worked  in  Cornwall  consists 
simply  of  granite  decomposed  in  situ.  The  decomposition  is  obviously 
due  to  the  action  of  highly  heated  vapours  coming  from  the  more  deeply 
seated  and  perhaps  unconsolidated,  or  only  partially  consolidated, 
portion  of  the  plutonic  mass. 


CHAPTER    IX 

INCLINATION    AND   CURVATURE   OF   STRATA 

Dip — Apparent  and  True.  Terminal  Curvature.  Outcrop  influenced  by 
Angle  of  Dip  and  Form  of  Ground.  Strike.  Curvature  of  Strata— 
Monoclinal  Folds,  Quaquaversal  and  Centroclinal  Folds,  Normal  or 
Symmetrical  Folds,  Unsymmetrical  Folds,  Inversion,  Recumbent 
Folds,  Fan-shaped  Structure,  Contorted  Strata,  Origin  of  Folds. 

IN  considering  the  formation  of  rock-beds,  some  incidental 
reference  was  made  to  the  fact  that  strata,  which  must 
originally  have  been  horizontally  disposed,  are  now  frequently 
inclined,  and  even  flexed,  folded,  and  contorted.  These  and 
other  superinduced  structures  now  fall  to  be  described  in 
more  or  less  detail. 

Dip.— The  dip  is  the  inclination  of  beds  down  into  the  earth, 
and  is  measured  in  degrees  by  the  angle  between  the  plane 
of  the  strata  and  the  plane  of  the  horizon.  The  instrument 
employed  for  this  purpose  is  called  a  clinometer — a  graduated 
arc  with  pendulum.  For  general  use  it  is  convenient  to  have 
the  clinometer  combined  with  a  compass — with  the  latter 
one  takes  the  direction,  and  with  the  former  the  degree  or 
amount  of  dip.*  When  strata  are  so  exposed  that  the  line  of 
greatest  inclination  can  be  observed,  the  direction  and 
amount  of  dip  are  readily  ascertained.  If  the  surface  of  an 
exposed  bed  be  smooth  and  even  we  have  only  to  place  the 
clinometer  upon  it,  taking  care  that  the  edge  of  the  instrument 
is  arranged  in  the  direction  of  greatest  slope  (i.e.  the  direction 
in  which  water  would  flow  if  poured  upon  the  surface),  and 
that  the  pendulum  is  swinging  freely.  The  pendulum  points, 
of  course,  to  the  degree  or  amount  of  dip.  If  the  surface  be 
not  very  smooth,  one  may  lay  one's  hammer  or  walking-stick 
upon  the  rock  in  the  line  of  dip,  and  thus  provide  a  longer 
*  See  Appendix  E. 


127 


128 


STRUCTURAL  AND  FIELD  GEOLOGY 


edge  on  -which  to  place  the  clinometer  —  the  object  being,  of 
course,  to  get  as  true  an  average  as  possible  for  the  whole 
surface.  But  to  insure  this,  it  is  always  advisable  to  check 
the  result  thus  obtained  by  taking  the  angle  of  dip  at  a  little 
distance  from  the  section  exposed.  To  do  so,  the  observer, 
standing  back  from  the  section,  holds  the  clinometer  within 
a  short  distance  of  his  eye,  and  in  such  a  position  that  the 
straight  edge  of  the  instrument  shall  coincide  with  the  lines 
of  the  dipping  strata.  The  distance  at  which  one  should 
make  an  observation  of  this  kind  will  depend  largely  on  the 
height  of  the  exposed  section.  If  the  height  be  only  a  few 
yards  the  dip  may  be  measured  at  no  greater  distance  than 
the  height.  But  if  the  section  be  much  higher  the  observer 


FIG.  10. — DIP  AND  STRIKE  OF  STRATA. 

must  stand  proportionally  further  back — the  object  being  to 
make  the  edge  of  the  clinometer  coincide  with  as  long  a 
stretch  of  the  lines  of  bedding  as  possible.  In  this  way  we 
usually  get,  by  means  of  one  observation,  a  more  reliable 
average  than  we  should  if  we  had  taken  the  average  of  twenty 
observations  made  by  placing  the  clinometer  directly  on  the 
rock-surface.  Even  in  the  case  of  false-bedded  strata,  it 
is  often  possible,  by  standing  well  back  from  the  section,  to 
get  a  good  average  dip  for  the  whole  series.  But  when  the 
actual  surfaces  of  the  bedding-planes  are  not  visible,  the 
beginner  may  easily  be  deceived  as  to  the  true  position  of 
the  strata.  The  lines  of  bedding  which  are  seen  travers- 
ing the  face  of  a  cliff  do  not  necessarily  indicate  the  true 
direction  and  amount  of  dip.  Beds  that  are  really  inclined 
may  even  appear  to  be  horizontal.  In  the  accompanying 


XXIX. 


[To  face  page  128. 


[To  face  page  129. 


CURVATURE  OF  STRATA  129 

section,  for  example  (Fig.  11),  the  beds  at  a  seem  to  be 
horizontal,  when  in  reality  they  dip  at  a  considerable  angle, 
as  shown  at  b — where  the  cliff  runs  in  the  direction  of  the 
true  dip — the  direction  and  amount  of  which  can  therefore  be 
readily  determined.  Not  infrequently,  however,  cliffs  and 
other  cuttings  or  sections  traverse  the  dip  of  the  strata 
obliquely,  and  when  such  is  the  case,  the  apparent  dip  shown 
by  the  edges  of  the  exposed  beds  does  not  indicate  either  the 
exact  direction  or  the  full  amount  of  the  true  dip,  which  is 
always  greater  than  that  of  the  apparent  dip.  When  the 
observer  suspects  that  the  inclinations  exposed  in  two 
adjacent  sections  are  only  apparent  dips,  he  may  yet  find  the 
true  direction  by  the  geometrical  method  referred  to  in 
Appendix  C.  As  a  rule,  however,  an  apparent  dip  can  rarely, 


FIG.  ii. — APPARENT  AND  TRUE  DIP. 

if  ever,  deceive  one  who  is  not  content  to  view  sections  from  a 
distance.  Close  examination  will  rarely  fail  to  discover  on 
even  the  smoothest  of  cliff-faces,  irregularities  —  ledges, 
depressions,  entering  and  re-entering  angles,  etc. — in  one  or 
other  of 'which  the  upper  or  under  surfaces  of  the  bedding- 
planes  are  almost  sure  to  be  disclosed. 

In  hilly  and  mountainous  tracts,  the  exposed  ends  of 
strata  often  present  a  fallacious  appearance  of  dip,  which  has 
occasionally  led  to  mistakes.  The  appearance  referred  to  is 
known  as  "terminal  curvature"  or  "surface  creep,"  and  is 
illustrated  in  the  accompanying  figures  (Figs.  I2a,  \2b].  An 
observer  ascending  the  mountain  slopes  shown  in  the 
diagrams  might  quite  well  be  deceived  by  the  apparent  dip 
of  the  beds,  if  it  did  not  so  frequently  happen  that  the  true 
dip  of  the  rocks  in  such  a  region  is  usually  exposed  in 
numerous  torrent-tracks  and  gullies.  The  origin  of  terminal 
curvature  is  obvious  enough — being  solely  the  result  of 

I 


130 


STRUCTURAL  AND  FIELD  GEOLOGY 


weathering.  Rain-water  insinuates  itself  between  the  bedding- 
planes,  and  the  strata  are  thus  exposed  not  only  to  its 
chemical  and  mechanical  action,  but  to  the  more  powerful 
action  of  frost.  The  latter  tends  to  force  the  beds  apart — 


FIG.  1 2a.—  TERMINAL  CURVATURE  IN  STEEPLY  INCLINED  STRATA. 

movement  taking  place  chiefly  in  the  line  of  least  resistance, 
which,  of  course,  is  downhill.  In  this  way  the  edges  of  the 
beds  are  gradually  turned  over,  so  as  to  present  an  apparent 
dip  which  may  be  exactly  opposite  to  the  true  inclination  of 
the  strata.  In  high-lying  districts  this  inverting  process  is 


FIG.  i2(5. — TERMINAL  CURVATURE  IN  HORIZONTAL  AND  INCLINED  STRATA. 

often  aided  by  the  movement  of  massive  heaps  of  snow,  and 
by  the  downward  creeping  of  water-saturated  sheets  of  earthy 
rock-debris,  which  tend  to  drag  forward  the  edges  of  the  beds 
in  the  direction  of  movement. 

Outcrop  is  the  term  applied  to  the  edges  of  the  strata 
which  appear  at  the  surface.  An  outcrop  may  be  exposed 
or  visible,  or  it  may  be  covered  and  concealed  under  younger 
accumulations  (see  Figs.  13,  14).  As  a  rule,  the  direction  of 
the  outcrop  is  influenced  partly  by  the  inclination  of  the 
strata  and  partly  by  the  form  of  the  ground.  When  the 
beds  are  quite  horizontal,  every  change  in  the  configuration 


CURVATURE  OF  STRATA 


131 


of  the  surface  must  affect  the  direction  of  the  outcrops,  which 
in  such  a  case  behave  as  contour-lines  or  lines  of  equal 
elevation,  and  follow  all  the  irregularities  of  the  ground. 
When  strata  are  gently  inclined,  the  outcrops  are  also  strongly 
b 


FIG.  13. — OUTCROPS  CONCEALED  UNDER  BOULDER-CLAY,  /;. 

affected  by  the  shape  of  the  surface,  but  this  influence 
gradually  lessens  as  the  angle  of  dip  increases,  the  outcrops, 
as  the  beds  approach  verticality,  becoming  more  and  more 
persistent  in  direction,  and  being  less  and  less  modified  by 
changes  in  the  form  of  the  ground.  When  the  strata  are 


FIG.  14.— OUTCROPS  CONCEALED  UNDER  OVERLYING  STRATA. 

actually  vertical  or  standing  on  end,  the  outcrops  then  run 
in  straight  lines  across  hill  and  dale,  being  practically  inde- 
pendent of  the  surface  features. 

A  little  consideration  will  show  that  the  breadth  or  width 
of  an  outcrop  must  similarly  be  influenced  by  the  angle  of  dip 


FIG.  15.— WIDTH  OF  AN  OUTCROP  AFFECTED  BY  ANGLE  OF  DIP. 

and  the  form  of  the  ground.  In  the  case  of  horizontal  beds, 
the  uppermost  stratum,  although  it  may  be  quite  thin,  must 
frequently  occupy  a  relatively  wide  area  and  form  a  broad 
outcrop.  With  inclined  strata,  it  is  obvious  that  the  outcrops 
will  be  broad  or  narrow  according  as  the  dip  is  low  or  high 


132 


STRUCTURAL  AND  FIELD  GEOLOGY 


the  lower  the  angle  of  dip,  the  wider  the  outcrop  (see  Fig.  15). 
As  the  dip  increases,  the  width  of  the  outcrops  gradually 
diminishes  until  the  strata  become  vertical,  and  then  the 
width  of  outcrop  can  be  no  more  than  the  actual  thickness 
of  the  beds. 

The  accompanying  diagram  (see  Fig.  16)  may  suffice  to 
illustrate  how  the  width  of  an  outcrop  is  affected  by  surface 
features.  The  beds  I,  2,  3,  as  seen  in  section,  are  of  equal 
thickness,  but  their  outcrops,  owing  to  the  shape  of  the 
ground,  vary  much  in  width.  Bed  I ,  appearing  upon  relatively 
flat  land,  yields  a  broad  outcrop  ;  bed  3,  forming  the  surface 
of  a  gently  inclined  plateau,  covers  a  much  wider  area ;  while 


FIG.  16.— WIDTH  OF  OUTCROP  AFFECTED  BY  FORM  OF  GROUND. 

bed  2,  coming  out  on  a  steep  slope,  would  show  upon  a  map 
an  outcrop  somewhat  narrower  than  the  true  thickness  of 
the  stratum. 

Strike  is  a  line  drawn  exactly  transverse  to  the  dip. 
Thus  beds  with  an  east  dip  have  a  north  and  south  strike. 
The  strike  rarely  coincides  with  the  outcrop ;  usually  it  only 
does  so  in  the  case  of  vertical  strata,  the  outcrops  of  which 
are  not  affected  by  the  form  of  the  ground.  Now  and  again, 
however,  when  the  edges  of  strata  inclined  at  any  angle  crop 
out  upon  a  level  plain,  outcrop  and  strike  may  coincide.  The 
term  strike  is  generally  used  by  geologists  when  they  are 
referring  to  the  average  direction  of  an  outcrop.  Thus  a 
great  succession  of  strata  having  a  persistent  or  dominant 
dip,  say  towards  the  north,  are  said  to  have  an  east-and-west 
-strike,  no  matter  how  sinuous  and  irregular  the  outcrops 


CURVATURE  OF  STRATA 


133 


may  be  (Fig.   17).      Again,  when  two   series   of  strata   with 
discordant   dips   occur   in   juxtaposition,  the  one  set  is   said 


FIG.  18.— STRATA  STRIKING  AT  EACH 
OTHER. 


FIG.  17. — OUTCROP  AND  STRIKE. 

to  strike  at  or  against  the  other.  The  conditions  referred  to 
are  shown  in  the  ground-plan  (Fig.  18),  where  the  cause  of 
the  discordance  is  the 
presence  of  a  fault  (see 
Chap.  XL). 

Curvature  of 
Strata.— Inclined  beds 
are  usually,  but  not 
always,  parts  of  large 
curves  or  undulations. 
Under  certain  condi- 
tions, as  in  the  case  of 
deltas,  we  may  have  a 

succession  of  imbricating  and  interosculating  beds,  all  the 
members  of  the  series  showing  a  general  dip  in  the  direction 
followed  by  the  sediment-transporting  current.  Further,  it  is 
obvious  that  the  lower  beds  of  a  great  succession  of  strata 
accumulated  in  a  basin-shaped  depression,  must  be  more 
or  less  inclined,  according  as  the  floor  of  the  basin  shelves 
rapidly  or  gradually.  But  with  continuous  sedimentation  the 
inequalities  of  lake-bottom  and  sea-floor  must  eventually  be 
obliterated,  and  the  bulk  of  the  deposits  come  to  occupy  an 
approximately  horizontal  position.  There  is  little  reason  to 
doubt,  therefore,  that  all  the  great  systems  of  marine 
sedimentary  strata  were  originally  for  the  most  part  arranged 
in  successive  horizontal  layers  and  sheets.  With  such  excep- 
tions as  those  referred  to  above,  the  inclined  position  which 
strata  now  so  frequently  occupy  must  be  due  to  subsequent 
crustal  deformation.  Strata  originally  horizontal  have  been 
thrown  into  gentle  undulations  and  sharper  folds,  and  the 


134 


STRUCTURAL  AND  FIELD  GEOLOGY 


tops  of  such  folds  and  undulations  having  been  gradually 
denuded  away,  the  truncated  ends  of  the  strata  now  crop  out 
at  the  surface. 

As  might  have  been  expected,  folds  present  every  degree 
of  complexity.  Some  are  broad  and  open,  others  are  narrow 
and  compressed ;  in  some  the  strata  are  but  slightly  disturbed 
— they  simply  rise  and  fall  in  gentle  undulations — in  others 
the  beds  may  be  twisted,  contorted,  and  confused  in  the  most 
extraordinary  manner. 

Monoclinal  Flexure. — The  simplest  kind  of  flexure  is 
the  monocline.  This  structure  is  met  with  chiefly  in  regions 
of  horizontal  or  gently  inclined  strata.  It  may  be  shortly 
defined  as  a  sudden  dip  or  abrupt  increase  of  dip  followed 
by  an  equally  abrupt  return  to  the  former  horizontal  or 
gently  inclined  position  (see  Fig.  19).  Frequently  the  strata 


FIG.  19. — MONOCLINAL  FLEXURE. 

in   the   limb   of  a   monoclinal    fold    appear   attenuated   (see 
Fig.  20),  as  if  they  had  either  been  laterally  compressed    or 


FIG.  20. — MONOCLINAL  FOLD  SHOWING  THINNING  OF  BEDS  IN  THE  FOLD. 

drawn  out.  As  we  shall  see  later  on,  this  attenuation 
becomes, still  more  pronounced  until  the  limb  of  the  flexure 
vanishes  and  is  replaced  by  a  fault  or  dislocation. 

Quaquaversal  and  Centroclinal  Folds,— Now  and  again, 


CURVATURE  OF  STRATA 


135 


n 


regions  of  gently  inclined  strata,  we  encounter  dome- 
shaped  and  basin-shaped  structures.  When  the  strata  are 
dome-shaped  they  are  said  to  have  a  quaquaversal  dip,  i.e. 
they  are  inclined  outwards  in  all  directions  from  a  common 
point  (see  Fig.  21).  The  converse  of  this  structure  is  seen  in 
a  centroclinal  fold  —  the  beds  dipping  inwards  from  all  direc- 
tions towards  a  central  point  (see  Fig.  22).  But  symmetrical 


FIG.  21. — QUAQUAVERSAL  FOLD. 

a,  ground-plan ;  b,  section  along  line  A — B. 


FIG.  22. — CENTROCLINAL  FOLD. 

a,  ground-plan  ;  b,  section  along  line  A — B. 


or  complete  quaquaversal  and  centroclinal  folds  are  of 
somewhat  rare  occurrence,  and  may  be  looked  upon  as 
accidental  modifications  of  normal  anticlinal  and  synclinal 
folds. 

Normal  or  Symmetrical  Folds. — Strata,  as  a  rule,  are 
folded  along  axes.  This  is  true  of  the  simplest  flexures 
(monoclines),  and  of  all  the  more  complex  folds  to  be 
described.  The  axes  or  axial  planes  of  normal  folds  are 
approximately  vertical,  and  usually  extend  in  straight  or 
gently  curving  lines.  They  vary  much  in  length — from  a 
hundred  yards  or  less  to  many  miles.  When  the  strata  dip 


136  STRUCTURAL  AND  FIELD  GEOLOGY 

away  from  such  an  axis  on  either  side  at  approximately  the 
same  angle,  the  structure  is  known  as  a  Symmetrical  Anticline 
or  Saddleback  (Plate  XXIX.).  The  converse  structure,  in 
which  the  strata  dip  in  from  either  side  at  equal  angles  to  a 
central  axis,  is  termed  a  Symmetrical  Syncline  or  Trough 
(see  Fig.  23).  When  the  inclination  of  the  strata  is  moderate, 
individual  anticlines  and  synclines  do  not  usually  extend  for 
any  distance.  A  wide  region  of  gently  undulating  strata 
often  recalls  the  appearance  presented  by  a  slightly  rumpled 
tablecloth — in  which  the  individual  wrinkles,  sometimes  short, 
sometimes  long,  succeed  each  other  at  inconstant  intervals ; 
and  while  tending,  perhaps,  to  run  in  a  particular  direction, 
are  yet  frequently  straggling  and  irregular.  But  when  the 
strata  are  inclined  at  higher  angles,  anticlinal  and  synclinal 


a 
FIG.  23. — NORMAL  OR  SYMMETRICAL  FOLDS. 

a,  anticline ;  s  s,  synclines. 

folds  are  apt  to  extend  for  longer  distances,  and  to  preserve 
their  parallelism  more  or  less  persistently.  When  folding  is 
well  developed,  it  is  often  possible  to  follow  the  axes  of 
individual  anticlines  and  synclines  throughout  their  whole 
extent.  Each  fold  begins  at  zero — forming,  at  first,  a  quite 
insignificant  "lirk"  or  crease;  little  by  little,  as  we  follow 
the  axis,  the  dip  of  the  strata  augments  until  in  a  longer  or 
shorter  distance  the  maximum  inclination  is  reached,  after 
which  the  dip  usually  begins  to  decrease,  and  finally  the  fold 
dies  away.  Not  infrequently,  however,  folds  increase  and 
diminish  in  an  irregular  manner — a  great  system  of  parallel 
anticlines  and  synclines  often  consisting  of  a  series  of 
dovetailed  and  interlocked  folds  of  variable  width  and 
extent. 

When  folded  strata  have  been  for  a  long  period  exposed 
to  denudation,  the  original  crowns  or  crests  of  the  anticlines 
have  invariably  disappeared — the  ridges  have  been  gradually 


PLATE  XXXI, 


o 

CO    • 

W 


PLATEXXXII. 


[Between  panes  136  and  137. 


CURVATURE  OF  STRATA  137 

lowered  by  denudation.  On  the  other  hand,  the  synclinal 
structure  has  evidently  offered  greater  resistance  to  the  forces 
of  decay,  for  not  infrequently  we  find  that  hills  are  built  up 
of  trough-shaped  strata.  But  although  the  original  tops  of 
all  anticlinal  ridges  have,  as  a  rule,  disappeared,  and  synclinal 
troughs  have  also  been  reduced,  geologists  still  speak  of  these 
structures  as  if  they  were  perfect  folds.  Fig.  24  represents 


FIG.  24. — MODEL  OF  DENUDED  SYNCLINAL  AND  ANTICLINAL  FOLDS. 

a— 6,  a — 6,  axes  of  folds. 

the  model  of  two  synclines  with  intervening  anticline — the 
arrows  indicating  the  direction  of  the  dip.  The  dotted  lines 
(a  b)  are  the  axes  of  the  folds.  The  section  shows  the 
geological  structure,  and  the  relation  of  that  structure  to  the 
surface  features. 

Unsymmetrical  Folds. — When  the  axial  plane  of  a  fold 
is  inclined  at  any  angle  from  the  vertical,  the  fold  is  said  to 
be  unsymmetrical.  The  inclination  may  be  very  slight — so 
slight  that  the  strata  on  either  side  of  the  axis  may  have 
much  the  same  angle  of  dip ;  or  the  axial  plane  may  depart 
so  far  from  the  vertical  as  to  be  actually  horizontal,  and  the 
fold  then  lies  on  its  side.  Between  these  extremes  every 
degree  is  encountered.  As  a  rule,  unsymmetrical  folds  are 
closely  compressed,  the  limbs  flattened  against  each  other, 
and  the  crowns  of  the  arches  usually  somewhat  pointed. 
Some  typical  forms  are  represented  in  the  accompanying 
illustrations  (Figs.  25-30).  When  the  axial  plane  is  so  much 
inclined  that  one  limb  of  the  fold  becomes  doubled  under  the 
other,  we  have  the  structure  known  as  an  Overfold  (see  Fig. 


138 


STRUCTURAL  AND  FIELD  GEOLOGY 


25).     In  a  fold  of  this  kind  the  strata  which  form  its  lower  limb 
are  necessarily  turned  upside  down,  and  hence  the  structure 


FIG.  25.— UNSVMMETRICAL  FLEXURES:  OVERFOLDS. 

is  frequently  termed  Inversion.     Unsymmetrical  and  closely 
compressed  symmetrical  folds  not  infrequently  occur  together, 


FIG.  26. — ISOCLINAL  FOLDS. 


but   in    regions   of  highly  inclined    and    vertical   strata    the 
flexures   are   usually   unsymmetrical,   and   for  the  most  part 


FIG.  27. — ISOCLINAL  FOLDS,  MUCH  DENUDED. 

are   overfolds.       In   such   cases   the   successive   axial    planes 
very  often  incline  for  long  distances  in  the  same  direction — 


CURVATURE  OF  STRATA 


139 


the  flexures  which  show  this  arrangement  being  termed 
Isoclinal  folds  (see  Figs.  26,  27).  As  the  original  crowns 
of  the  anticlines  have  invariably  been  removed,  the  truncated 
beds  present  the  deceptive  appearance  of  a  great  series  of 
strata,  all  dipping  at  high  angles  in  the  same  direction.  In 
reality,  however,  as  a  glance  at  Fig.  27  will  show,  the  same 
beds  are  again  and  again  repeated,  so  that  the  series  is  not 


FIG.  28. — RECUMBENT  FOLD. 
by  any  means  so  thick  as  it  might  at  first  seem  to  be. 


This 


structure  is  very  well  developed  in  the  Southern  Uplands  of 
Scotland.  Recumbent  fold  (Fig.  28)  is  the  name  given  to  a 
flexure,  the  axial  plane  of  which  approaches  horizontality. 


FIG.  29.— ANTICLINAL  DOUBLE-FOLD. 

It  is  a  structure  of  frequent  occurrence  in  regions  of  highly 
convoluted  strata,  but  is  not  so  common  as  the  ordinary 
overfold.  Another  structure,  particularly  characteristic  of 
the  more  highly  disturbed  portions  of  the  earth's  crust,  is 
that  known  as  an  Anticlinal  Double-fold  (see  Fig.  29).  In 


140 


STRUCTURAL  AND  FIELD  GEOLOGY 


this  structure  two  unsymmetrical  synclines  approach  each 
other  from  opposite  directions,  while  in  the  intervening 
space  the  strata  are  arched  into  a  great  anticline.  The 
crown  of  the  anticline  has  invariably  disappeared,  so  that  the 
truncated  strata  are  seen  to  dip  in  from  both  sides  towards 
the  axial  plane.  Since  the  beds  within  the  anticline  are 
much  compressed  below  while  they  open  out  above,  they 
present  the  appearance  known  as  fan-shaped  structure  (see 
Fig.  30). 


FIG.  30.— SECTION  ACROSS  MOUNT  BLANC,  SHOWING 
FAN-SHAPED  STRUCTURE. 

All  the  several  kinds  of  unsymmetrical  folds  described  in 
the  preceding  paragraph  occur  in  regions  which  have  been 
subjected  to  some  dominant  movement  of  the  crust — either 
of  elevation  or  depression.  When  a  broad  zone  has  bulged 
up  under  lateral  pressure  to  form  a  mountain  chain,  we  have, 
as  in  the  Alps,  one  great  arch  composed  of  numerous  sub- 
ordinate wrinkles  or  minor  folds  and  flexures.  A  complex 
arch  of  this  kind  is  termed  an  Anticlinorium  (see  Fig.  31). 


FIG.  31. — DIAGRAM  OF  AN  ANTICLINORIUM. 

If  the  arch  be  simple — a  broad  anticlinal  fold  with  no 
conspicuous  wrinklings  or  flexures — it  is  known  as  a 
Geanticline.  The  converse  structure,  resulting  from  the 
depression  of  a  broad  zone,  is  termed  a  Synclinorium,  when 


CURVATURE  OF  STRATA 


141 


the    great    trough     is    complicated    by    many     subordinate 
foldings,  and  a  Geosyncline  when  the  trough  is  simple. 


FIG.  32. — DIAGRAM  OF  A  SYNCLINORIUM. 

Contorted  Strata. — When  strata  are  so  unsym metrically 
and  abundantly  folded  that  it  becomes  difficult  or  impossible 
to  trace  out  the  individual  flexures  and  crumplings — the 
whole  forming  an  irregular  complex  of  folds — they  are  said 
to  be  contorted  (see  Plates  XXX.-XXXIL).  Such  contorted 
rocks  are  frequently  associated  with  the  several  kinds  of  un- 
symmetrical  folds  described  in  the  preceding  section.  In  all 
highly  folded  and  corrugated  strata,  the  rocks  have  obviously 
been  subjected  to  great  compression.  This  is  seen  in  the 
peculiar  thinning  and  thickening  undergone. by  the  strata — 
the  beds  becoming  attenuated  in  the  limbs  of  the  folds,  and 
swelling  out  again  in  the  cores  of  the  arches  and  troughs 
(see  Plates  XXXI.,  XXXII).  It  would  seem,  in  fact,  as  if, 
under  compression,  the  solid  rocks  had  been  compelled  to 
yield  and  to  behave  like  plastic  bodies.  The  evidence  of 
such  shearing  and  flowing  is  conspicuous  not  only  in  the 
larger  folds,  but  in  the  smallest  crumplings  visible  to  the 
naked  eye.  Indeed,  when  the  rocks  are  sliced  and  examined 
under  the  microscope,  they  continue  to  show  precisely  the 
same  structures.  It  is  thus  no  exaggeration  to  say  that  folds 
vary  in  dimensions  from  great  flexures  measuring  hundreds 
of  yards  across,  down  to  puckerings  and  crumplings  so 
minute  that  they  only  become  visible  under  the  microscope 
(see  Plate  V.  4). 

If  in  the  case  of  contorted  strata  the  individual  beds  and 


142  STRUCTURAL  AND  FIELD  GEOLOGY 

their  subordinate  laminae  have  become  distorted,  it  is  not 
surprising  that  their  individual  constituents  should  similarly 
yield  evidence  of  compression.  Thus  the  rounded  stones  of  a 
conglomerate  (Plate  XXI.  3)  are  often  flattened  against  each 
other  and  drawn  out  into  elliptical  or  lenticular  forms,  while 
fossils  are  frequently  distorted  in  like  manner.  This  process 
of  deformation  has  often  proceeded  so  far  as  to  result  in  the 
more  or  less  complete  alteration  of  the  rocks,  the  original 
characters  being  either  much  obscured  or  even  entirely 
obliterated.  But  the  further  consideration  of  such  changes 
must  be  deferred  until  we  come  to  discuss  the  phenomena  of 
slaty  cleavage  and  metamorphism. 

Origin  of  Folds. — The  various  folds  described  above  are  obviously  the 
result  of  lateral  compression  brought  about  by  the  sinking  of  the  superficial 
crust  of  the  globe  upon  the  cooling  and  contracting  interior.  If  we  think 
of  it,  there  must  be  a  gradual  passage  downwards  from  the  cooled  crust 
into  the  still  uncooled  nucleus.  At  some  depth  from  the  surface,  therefore, 
a  level  will  be  reached  at  which  the  interior  has  not  yet  begun  to  cool 
and  contract.  Theoretically,  we  may  consider  all  the  matter  above  that 
level  as  constituting  the  crust.  The  lower  section  of  the  crust,  reposing 
immediately  upon  the  uncooled  nucleus,  is  cooling  and  therefore  con- 
tracting, and  must  obviously  be  in  a  state  of  tension  to  which  it  will 
seek  to  yield  by  rupturing.  Not  that  fissures  or  rents  will  actually  be 
formed,  for  the  enormous  compression  exerted  by  the  upper  crustal  layers 
will  necessarily  prevent  anything  of  the  kind  taking  place.  The  stretching 
of  the  crust  by  lateral  tension  must  diminish  upwards,  until  a  level  is 
attained  where  it  will  cease  altogether.  Above  that  level  the  crust  is 
no  longer  in  a  state  of  lateral  tension,  but  in  one  of  lateral  compression — 
it  is  not  stretching  but  shrivelling.  And  the  lateral  compression  which 
causes  the  superficial  crustal  shell  to  shrivel  increases  upwards,  and  is 
therefore  greatest  at  the  surface.  There  are  thus  two  kinds  of  contraction 
to  which  the  crust  is  subjected,  namely,  circumferential  below  and  radial 
above.  When  the  former  is  in  excess,  stretching  with  tendency  to  rupture 
is  most  marked  ;  where  the  latter  prevails,  compression  is  dominant  ; 
where  the  one  equals  the  other  there  is  no  strain.  It  is  therefore  only 
the  crustal  shell  above  this  neutral  zone  or  "  level-of-no-strain  "  which  is 
liable  to  become  folded.  How  thick  that  shell  may  be  we  do  not  know, 
but  as  the  folded  rocks  in  some  mountain  chains  reach  a  thickness  of 
ten  miles  or  more,  the  upper  crustal  shell  must  be  of  that  thickness  at 
least.  Although  folded  strata  are  met  with  very  generally,  in  Old  and 
New  Worlds  alike,  nevertheless  we  now  and  again  enter  regions  of  great 
extent,  over  which  the  strata  have  retained  their  original  horizontal 
arrangement.  It  is  notable,  further,  that  while  gently  undulating  strata 
often  extend  throughout  vast  areas,  highly  folded  and  contorted  rocks 
tend  to  occur  in  zones  or  belts.  It  would  thus  seem  that  the  earth's 


CURVATURE  OF  STRATA  143 

crust  yields  unequally  to  the  lateral  compression  induced  by  its  sub- 
sidence on  the  cooling  and  contracting  interior.  The  younger  mountain 
ranges  of  the  globe — the  Alps,  the  Himalayas,  the  Andes,  the  Rockies, 
etc. — are  composed  essentially  of  highly  disturbed  and  complexly  folded 
and  contorted  rocks,  and  are  believed,  therefore,  to  indicate  zones  of 
weakness,  along  which  relief  from  pressure  has  been  readily  obtained. 
Similarly,  much  more  ancient  zones  of  steeply  flexed  and  folded  rocks, 
occurring,  it  may  be,  in  low-lying  regions,  show  geologists  where  gigantic 
mountains  formerly  existed,  and  assure  them  that  from  the  earliest  times 
the  crust  has  found  relief  from  lateral  pressure  by  buckling  up  along  lines 
of  weakness. 

Some  further  remarks   on  folding  and  its  results  will  be  found  in 
the  following  chapter. 


CHAPTER  X 

JOINTS 

Joints,  Close  and  Gaping.  Joints  in  Bedded  Rocks — Master-joints,  Dip- 
and  Strike-joints.  Joints  in  Igneous  Rocks — in  Granitoid  Rocks, 
Prismatic  Joints.  Joints  in  Schistose  Rocks.  Slickensides.  Origin 
of  Joints — Contraction,  Expansion,  Crustal  Movements. 

JOINTS  are  superinduced  divisional  planes  which  traverse 
rocks  in  different  directions  and  at  various  angles,  so  as  to 
allow  of  their  ready  separation  into  larger  and  smaller  blocks 
and  fragments  of  regular  or  irregular  shape.  The  faces  of  a 
joint  are  generally  smooth  and  flat,  but  in  certain  cases  (as  in 
many  crystalline  igneous  rocks),  they  are  often  somewhat 
curved.  In  fresh,  unweathered  rocks,  joints  are  usually 
inconspicuous,  the  faces  being  sometimes  in  such  close 
apposition  that  the  fissure  can  hardly  be  detected.  The 
presence  of  even  the  closest  joints,  however,  is  often  betrayed 
by  the  alteration  of  the  rock  induced  by  percolating  water — 
the  degree  of  alteration  naturally  depending  to  a  large  extent 
upon  the  character  of  the  rock.  In  the  case  of  red  sandstone, 
for  example,  the  position  of  the  joints  is  frequently  indicated 
by  more  or  less  vertical  lines  and  bands  of  bleached  rock. 
In  limestone,  again,  the  joints  tend  to  gape,  as  might  have 
been  expected  from  the  ease  with  which  that  rock  is  dis- 
solved by  acidulated  water.  The  faces  of  joints  are  frequently 
coated  with  a  pellicle  of  brown  or  yellow  iron-oxide ;  or  with 
other  depositions  from  aqueous  solution,  such  as  calcite, 
barytes,  quartz,  chalcedony,  etc.  Gaping  joints  are  in  like 
manner  often  filled  with  similar  products — and  are  then 
described  as  veins,  which  may  vary  in  width  from  less  than  an 
inch  up  to  many  feet.  The  phenomena  of  mineral  veins, 

144 


PLATE  XXXIII. 


A. 


[To  face  page    144. 


PLATE  ^X?^Y;;  ;    ;  ^ 


[To  face  page  145* 


JOINTS  145 

however,  will   be   considered   in    a   later   chapter,   under   the 
general  head  of  Lodes. 

As  a  rule,  the  more  important  joints  in  solidified  rocks  of  all  kinds 
tend  to  be  somewhat  open,  or,  at  all  events,  are  most  readily  recognised 
at  and  for  some  distance  down  from  the  surface,  becoming  less  and  less 
conspicuous  as  they  are  followed  to  greater  depths.  It  is  impossible  to 
doubt  that  these  appearances  are  due  to  epigene  action,  the  influence  of 
which  must  gradually  die  out  downwards.  The  opening  of  the  joints  in 
readily  soluble  rocks  like  limestone,  may  be  safely  attributed  to  percolat- 
ing water  ;  but,  in  the  case  of  relatively  insoluble  rocks  the  fissures  can 
hardly  have  been  opened  by  the  same  means,  and  are  more  likely  to  have 
been  widened  by  changes  of  temperature.  In  temperate  latitudes, 
however,  diurnal  and  seasonal  changes  of  temperature  do  not  affect  rocks 
beyond  a  few  feet  from  the  surface,  and  can  scarcely  account,  therefore, 
for  the  phenomena  referred  to.  We  must  remember,  however,  that 
during  relatively  recent  geological  times  the  present  temperate  latitudes 
of  Europe  and  North  America  experienced  certain  remarkable  climatic 
vicissitudes — having  sometimes  been  subjected  for  lengthy  periods  to  the 
rigours  of  an  arctic  climate,  while  at  other  times  the  conditions  would 
seem  to  have  been  more  genial  than  they  are  now.  Under  such  alterna- 
tions of  cold  and  heat  the  rocks  could  hardly  fail  to  have  been  affected  to 
a  much  greater  depth  than  is  possible  at  present.  We  know  that  in  high 
latitudes  the  ground  is  permanently  frozen  to  a  depth  of  over  a  hundred 
feet,  and  that  the  heat  of  summer  suffices  to  thaw  only  a  thin  superficial 
stratum.  Were  glacial  conditions,  therefore,  again  to  supervene  in 
temperate  latitudes,  we  cannot  doubt  that  with  increasing  cold  frost  would 
penetrate  ever  deeper  and  deeper — the  rocks  contracting  and  all  moisture 
becoming  frozen  to  depths  approximating  those  reached  by  frost  in  sub- 
arctic regions.  With  the  gradual  return  of  genial  conditions  thawing  would 
ensue  until  no  part  of  the  ground  remained  permanently  frozen.  To  this 
process  of  alternate  freezing  and  thawing,  repeated  again  and  again 
throughout  the  long  glacial  cycle,  we  ought  perhaps  to  assign  the  open- 
ing up  of  joints  at  considerable  depths  from  the  earth's  surface. 

Joints  in  Bedded  Rocks.— Sedimentary  rocks  are  usually 
traversed  by  two  sets  of  joints,  perpendicular  to  the  planes  of 
bedding,  and  intersecting  each  other  at  approximately  right 
angles.  Not  infrequently  these  joints  run  roughly  parallel 
for  long  distances,  and  when  they  do  so  they  are  known  as 
Master-joints.  Usually,  however,  it  is  impossible  to  follow 
individual  joints  very  far,  and  the  parallelism  of  a  series  is 
only  approximate,  for  often  enough  one  joint  runs  into 
another.  In  most  cases,  indeed,  individual  joints  seem  to  die 
out  in  a  few  yards,  and  to  be  succeeded  after  a  longer  or 
shorter  interval  by  one  or  more  following  the  same  general 

K 


146  STRUCTURAL  AND  FIELD  GEOLOGY 

direction.  The  width  of  rock  between  adjacent  joints,  belong- 
ing to  the  same  parallel  series,  is  very  variable.  In  some 
cases  it  may  be  many  feet  or  even  yards ;  in  other  cases  it 
may  be  considerably  less  than  a  foot.  In  certain  thin-bedded 
strata,  for  example,  so  closely  set  are  the  joints  that  the  rock 
breaks  up  readily  into  small  cubes  and  parallelepipeds.  In 
addition  to  the  more  or  less  regularly  intersecting  main  joints, 
numerous  subordinate  joints  may  traverse  a  rock  in  many 
different  directions ;  and  when  such  is  the  case,  the  system  of 
main  joints  becomes  obscured  or  unrecognisable — the  irregu- 
larly fissured  rock  breaking  up  into  a  rubble  of  larger  and 
smaller  angular  fragments  (see  Plates  XXXIII.,  XXXIV.). 

When  bedded  rocks  are  inclined,  the  intersecting  main 
joints  are  known  as  dip-joints  and  strike-joints  respectively — 
the  former  running  in  the  direction  of  the  dip  or  inclination 
of  the  beds,  while  the  latter  cross  these  at  approximately 
right  angles.  Strike-joints  are  usually  more  pronounced  than 
dip-joints.  To  this  rule,  however,  there  are  many  exceptions — 
sometimes  the  latter  being  more  conspicuous  than  the  former, 
while  not  infrequently  the  one  set  is  hardly  better  developed 
than  the  other. 

The  joints  that  traverse  a  succession  of  alternating  strata 
of  limestone,  shale,  sandstone,  etc.,  are  often  interrupted  and 
sharply  shifted  to  the  side  as  they  pass  from  one  bed  to  another. 
Often,  indeed,  certain  beds  in  a  series  are  more  regularly 
divided  than  the  immediately  overlying  and  underlying  strata 
the  jointing  of  which  may  be  very  irregular,  and  more 
abundant  or  less  so  than  that  of  the  intermediate  strata. 
In  such  cases  the  divisional  planes  of  the  several  beds  appear 
to  be  more  or  less  independent. 

While  regular  jointing  may  be  met  with  in  all  kinds  of  derivative 
rocks,  it  is  best  displayed,  as  a  rule,  in  fine-grained  deposits  of  homo- 
geneous composition,  as  in  limestones,  freestones,  flagstones,  coal,  etc. 
Common  household  coal,  for  example,  is  divided  by  three  sets  of  planes 
disposed  at  right  angles  to  each  other :  namely  (a)  planes  of  bedding, 
with  a  dull  sooty  surface,  and  (b}  and  (c)  joint-planes  with  bright  surfaces. 
One  set  of  joints  (b]  runs  in  the  direction  of  the  inclination  or  dip  of  the 
bed,  and  is  termed  by  miners  the  "end  of  the  coal,"  the  other  set  (c] 
traverses  the  bed  at  right  angles  to  the  "end,"  and  is  known  as  the  "face 
or  cleat  of  the  coal."  The  "  face,"  therefore,  coincides  with  the  strike  of 
the  strata.  In  a  coal-mine  cross-galleries  are  driven  in  the  direction  of 


2    ^ 

£ 

z 
3 


[To  face  page  146. 


JOINTS  147 

the  "end"  (i.e.  with  the  dip  or  inclination  of  the  strata),  while  the 
working-galleries  are  driven  along  the  "face"  (i.e.  the  strike),  and  they 
necessarily  follow,  therefore,  a  level-course.  So  long  as  the  inclination 
of  the  strata  remains  constant  in  direction,  the  working-galleries  must 
follow  a  straight  line  at  right  angles  to  the  dip,  but  with  any  change  in 
the  direction  of  the  latter  there  will  necessarily  be  a  corresponding  change 
in  the  direction  of  the  "level-course." 

Excellent  examples  of  regular  jointing  are  exhibited  by  the  thick 
Carboniferous  Limestones  of  Ireland  and  England — the  main  or  master- 
joints  crossing  each  other  nearly  at  right  angles,  and  preserving  their 
direction  for  long  distances.  The  Old  Red  Sandstone  strata  of  N.E. 
Scotland,  which  are  so  finely  displayed  in  numerous  coast-sections,  afford 
equally  good  illustrations  of  the  same  structure. 

Joints  in  Igneous  Rocks  are  seldom  as  regularly 
arranged  as  those  of  sedimentary  strata ;  indeed,  they  are 
frequently  so  very  irregular  that  no  system  or  arrangement 
can  be  recognised.  In  other  cases,  however,  the  division- 
planes  show  a  modified  regularity,  following  determinate 
directions  like  the  master-joints  described  above ;  while  in 
yet  other  cases  they  are  so  symmetrically  disposed  as  to 
confer  a  prismatic  columnar  structure  on  the  rock  they 
divide. 

The  joints  in  granite  are  often  wonderfully  regular — two 
sets  of  vertical  or  steeply  inclined  division-planes  intersecting 
at  various  angles,  which  often  do  not  depart  widely  from 
right  angles.  The  rock  thus  tends  to  be  divided  into 
columnar  masses  that  vary  in  shape  according  to  the 
character  of  the  joints  (which  may  be  straight  or  curved), 
and  the  angle  at  which  they  intersect.  Sometimes  the 
joints  are  widely  separated,  so  that  large  monoliths  can  be 
obtained ;  at  other  times  they  are  so  closely  set  that  the 
rock  breaks  up  into  a  rubble  of  small  fragments.  The  main 
vertical  joints  are  often  accompanied  by  minor  irregular 
joints  the  presence  of  which  necessarily  prevents  the 
extraction  of  large  blocks.  In  addition  to  its  vertical 
division-planes  granite  often  exhibits  a  set  of  cross-joints, 
arranged  at  approximately  right  angles  to  the  others.  These 
cross-joints  may  be  horizontal  or  inclined,  and  often  give  the 
rock  a  kind  of  bedded  appearance,  at  least  towards  the  sur- 
face (Plate  XXXV.).  They  are  seldom,  however,  so  even  as 
planes  of  bedding.  Usually  they  are  somewhat  undulating, 


148  STRUCTURAL  AND  FIELD  GEOLOGY 

and  run  into  each  other  so  as  to  divide  the  rock  between 
the  vertical  joints  into  a  series  of  lenticular  and  inter- 
osculating  layers  or  sheets.  This  structure  when  viewed  from 
a  little  distance  sometimes  simulates  the  appearance  of 
false-bedding.  These  curious  joints  are  most  conspicuous 
towards  the  surface,  where  they  are  often  only  a  few  inches 
apart.  The  width  between  them,  however,  increases  with 
the  depth,  while  at  the  same  time  the  joints  become  closer 
and  more  discontinuous,  until  at  last  they  disappear.  The 
joints  in  question  are  thus  obviously  related  to  the  surface, 
and  this  relation  is  rendered  still  more  evident  by  the  fact 
that  they  are  always  approximately  parallel  to  the  surface. 
Thus,  when  the  ground  is  level  the  cross-joints  are  horizontal ; 
but  when  it  is  inclined,  the  joints  have  a  similar  dip.  In  a 
broad,  dome-shaped  mountain  of  granite,  for  example,  the 
rock  often  appears  to  consist  of  a  series  of  rudely  concentric 
shells,  which  at  the  summit  are  horizontal,  but  from  thence 
dip  outwards  in  all  directions,  coinciding  roughly  with  the 
average  fall  of  the  ground. 

Cross-joints  of  the  kind  described  above  are  not  confined  to  granite, 
but  occur  in  other  massive  eruptive  rocks.  They  have  been  observed, 
for  example,  in  some  syenites  and  quartz-porphyries,  but  are  seldom  so 
well  developed  (see  Plate  XXXVIII.).  [Even  homogeneous  sedimentary 
rocks,  such  as  limestone  and  freestone,  now  and  again  exhibit  a  similar 
structure,  which  cannot  be  mistaken  for  true  bedding.] 

The  columnar  structure  of  granitoid  rocks  due  to  jointing, 
is  by  no  means  so  well  developed  as  that  of  certain  other 
igneous  rocks.  Many  basalts,  for  example,  are  jointed  so 
symmetrically  that  the  rock  looks  like  an  organised  aggregate 
of  prismatic  columns  (see  Plate  XXXVII.).  When  this 
structure  is  fully  developed,  as  in  the  well-known  rocks  of 
Fingal's  Cave  and  the  Giant's  Causeway,  the  columns  tend 
to  assume  hexagonal  forms.  But  although  six-sided  columns 
are  common  enough,  yet  the  faces  of  the  prisms  are  seldom 
equally  developed,  while  many  columns  may  show  fewer  or 
more  faces  than  six,  so  that  trigonal,  tetragonal,  pentagonal, 
and  polygonal  forms  are  often  associated.  This  prismatic 
structure  is  always  developed  at  right  angles  to  the  planes  of 
cooling.  Hence,  when  the  rock  is  in  a  horizontal  position — 
the  upper  and  under  surfaces  being  planes  of  cooling — the 


JOINTS  149 

columns  are  vertical.  When,  on  the  other  hand,  the  molten 
rock  has  cooled  and  solidified  in  a  vertical  fissure,  the  walls 
of  this  fissure  form  the  cooling-planes,  and  the  columns  are 
therefore  horizontal.  In  the  case  of  intrusive  sills  or  sheets 
of  basalt,  the  columns  sometimes  extend  continuously  from 
one  cooling-plane  to  another  ;  in  dyke-like  intrusions,  however, 
they  are  usually  not  continuous,  but  separated  half-way  by 
an  irregular  line.  Now  and  again,  indeed,  small  dykes  and 
veins  of  basalt  are  wholly  composed  of  successive  thin  belts 
or  layers  of  prisms — the  prismatic  layers  being  separated  by  a 
series  of  roughly  parallel  fissures  (see  Figs.  73,  74,  p.  204).  In 
lavaform  rocks  the  columnar  structure  seems  likewise  to  be 
related  to  planes  of  cooling — the  columns  being  vertical  or 
inclined  according  as  the  rock  has  cooled  upon  a  horizontal 
or  an  inclined  surface.  Not  infrequently,  both  in  lava- 
form  and  intrusive  rocks,  the  columns  are  curved ;  and  in 
most  cases,  whether  curved  or  straight,  they  are  usually 
intersected  at  more  or  less  regular  intervals  by  transverse  or 
cross-joints,  which  in  some  few  cases  show  a  ball-and-socket 
arrangement — the  convex  surface  of  one  segment  fitting  into 
the  concave  surface  of  the  next  overlying  or  underlying  block. 
For  it  is  to  be  noted  that  the  convex  surfaces  of  the  segments 
in  adjacent  columns,  or  even  in  one  and  the  same  column,  do 
not  always  point  in  the  same  direction. 

The  columns  or  pillars  vary  much  in  size.  In  some  thin 
dykes  of  basalt  they  may  be  less  than  an  inch  in  diameter 
and  only  a  few  inches  in  length,  while  in  thick  sheets  and 
lava-flows  they  may  attain  a  thickness  of  I  or  2  feet,  and  a 
length  of  200  feet  or  more.  Prismatic  jointing,  although  as  a 
rule  best  developed  in  fine-grained  basic  igneous  rocks,  is  by 
no  means  confined  to  these,  for  it  is  often  well  developed  in 
andesites,  quartz-porphyries,  and  now  and  again  in  obsidian. 
[Neither  is  the  structure  in  question  confined  to  igneous  rocks. 
Even  sandstone  and  coal  occasionally  exhibit  a  superinduced 
prismatic  structure.  In  such  cases  the  rocks  have  been  in- 
fluenced by  the  presence  of  intrusive  igneous  masses.  Excel- 
lent examples  occur  in  the  Scottish  coalfields,  where  whole 
beds  of  coal  have  been  converted  into  a  kind  of  prismatic 
coke ;  while  at  and  near  their  contact  with  eruptive  rock  the 
sandstones  often  acquire  a  kind  of  rude  columnar  structure.] 


150  STRUCTURAL  AND  FIELD  GEOLOGY 

Joints  in  Schistose  Rocks.— As  schistose  or  foliated 
rocks  differ  much  in  composition  and  structure,  they  might 
have  been  expected  to  show  considerable  variety  in  the 
character  of  their  jointing.  Those  of  them  in  which  the 
foliated  structure  is  well  developed  are  occasionally  divided 
by  vertical  or  steeply  inclined  joints,  but  these  are  very  rarely 
arranged  symmetrically,  and  no  system  of  intersecting  joints 
like  those  of  sedimentary  strata  can  be  detected.  Now  and 
again,  however,  granitoid  gneiss  is  crossed  by  division-planes 
which  have  a  general  resemblance  to  the  joints  of  granite. 
But,  as  a  rule,  the  jointing  of  schistose  rocks  is  irregular  and 
capricious. 

Slickensides  (Plate  XXXVI.).— The  surfaces  of  joints  in 
all  kinds  of  rock  are  often  smoothed  and  marked  by  parallel 
ruts  and  striae,  as  if  the  opposite  rock  faces  had  been  ground 
and  rubbed  against  each  other.  These  slickensides,  as  they  are 
termed,  are  frequently  coated  with  a  skin  of  mineral  matter, 
which  naturally  shows  a  cast  of  the  opposite  joint  face,  and 
thus  has  the  appearance  of  having  itself  been  smoothed  and 
striated.  In  opening  up  a  joint  occupied  by  a  thin  vein  of 
mineral  matter,  it  is  often  possible  to  detach  complete 
segments  of  the  vein,  which  yield  a  cast  of  both  joint  faces. 
While  slickensides  are  not  confined  to  any  particular  areas, 
they  yet  tend  to  be  most  abundantly  developed  in  regions 
where  considerable  crustal  movement  or  rock-displacement 
has  taken  place.  They  often  occur,  for  example,  in  the  joints 
of  rocks  near  lines  of  fracture  and  dislocation  of  the  crust, 
and,  as  we  shall  learn  presently,  the  walls  or  faces  of  such 
dislocations  themselves  frequently  exhibit  the  same  smoothed 
and  striated  appearance. 

Origin  of  Joints.- — No  one  cause  suffices  to  explain  all  the  phenomena 
of  joints.  In  many  cases  we  can  hardly  doubt  that  these  superinduced 
division-planes  are  simply  fissures  of  retreat,  formed  during  the  consolida- 
tion and  solidification  of  the  rocks  in  which  they  occur.  Some  joints, 
again,  are  of  such  a  character  as  to  show  that  considerable  force  was 
required  for  their  production,  and  these  are  suggestive,  therefore,  of 
crustal  movements  of  some  kind.  The  precise  origin  of  others,  however, 
is  still  obscure.  The  more  obvious  causes  which  have  led  to  the  jointed 
structure  of  rocks  may  be  considered  under  the  headings  of  Contraction, 
Expansion,  and  Crustal  Movements. 

CONTRACTION. — Any  moist  and  plastic  rock,  such  as  clay,  necessarily 


PLATE  .XXXV1. 


i.  SLICKENSIDES,  PARTLY  COATED  WITH  MINERAL  MATTER  (WHITE). 


2.  SLICKENSIDES,  NOT  COATED. 


[To  face  page  150. 


JOINTS  151 

contracts  while  drying,  and  in  this  way  becomes  more  or  less  abundantly 
cracked  or  fissured.  Possibly  this  may  be  the  origin  of  many  of  the 
minor  or  subordinate  irregular  joints  of  sedimentary  strata,  but  it  does 
not  account  for  the  vertical  intersecting  joints  which  are  so  characteristic 
of  these  rocks.  The  passage  from  the  non-crystalline  to  the  crystalline 
condition  also  involves  contraction,  and  thus  we  may  believe  that  the 
crystallisation  of  certain  chemically  formed  deposits  may  have  been  the 
cause  of  their  jointed  structure.  To  the  same  cause  must  undoubtedly  be 
attributed  most  of  the  division-planes  occurring  in  crystalline  igneous 
rocks,  such  as  the  vertical  joints  in  granite,  and  the  prismatic  jointing  of 
basalt  and  other  eruptive  rocks.  The  cross-joints  of  granite,  however, 
cannot  be  accounted  for  in  this  way.  The  simple  fact  that  they  are 
present  only  in  the  upper  part  of  a  rock-mass  and  disappear  entirely  at 
lower  levels  suffices  to  show  that  they  have  not  the  same  origin  as  the 
vertical  joints  between  which  they  have  been  developed.  It  is  otherwise 
with  the  cross-joints  of  basalt,  etc.,  which  appear  to  be  of  the  same 
nature  as  the  prismatic  joints  with  which  they  are  associated — fissures  of 
retreat,  due  to  the  contraction  of  the  rock  in  cooling.  The  peculiar 
manner  in  which  basalt  and  many  other  igneous  rocks  weather  is 
somewhat  suggestive.  Prismatic  columns  which  have  been  long 
exposed  often  lose  their  angular  form,  the  individual  segments  or  blocks 
assuming  a  spheroidal  shape,  so  that  the  rock  appears  as  if  built  up  of 
vertical  rows  of  globular  ball-like  or  cheese-like  bodies.  Each  of  these 
spheroids  exfoliates  in  successive  concentric  shells — a  fraction  of  an  inch 
in  thickness — and  the  external  ones  may  be  readily  detached  by  the 
hammer.  The  shells,  however,  become  more  adherent  and  less 
conspicuous  as  we  penetrate  the  rock  until  they  cease  to  appear.  This 
peculiar  kind  of  weathering  is  not  confined  to  rocks  which  show  a 
prismatic  structure,  being  met  with  not  only  in  non-columnar  basalt  but 
in  many  other  igneous  rocks,  as  in  some  pitchstones,  granites,  diorites, 
porphyries,  etc.  The  weathering  often  proceeds  so  far  that  the  exfoliating 
crusts  break  down  into  a  kind  of  earthy  or  sandy  grit,  till  nothing  of 
the  original  rock  may  be  left  save  a  few  scattered  balls  or  cores.  It  is 
supposed  that  the  shell-like  structure  betrayed  by  weathering  is  really 
original,  the  centre  of  each  spheroid  having  been  a  centre  of  contraction. 
So  long  as  the  rock  is  fresh  the  structure  remains  invisible,  and  only 
becomes  apparent  when  weathering  supervenes.  This  is  a  plausible  or 
even  probable  explanation  of  the  phenomena,  but  it  does  not  quite  carry 
conviction.  The  perlitic  structure  of  glassy  rocks  (which  is  due  to  the 
presence  of  numerous  minute  and  roughly  concentric  cracks  produced 
during  cooling  and  contraction)  has  been  cited  as  an  example  on  a  small 
scale  of  that  spheroidal  structure  of  basalt,  etc.,  which  is  only  revealed  by 
weathering.  All  one  can  say  is,  that  nothing  comparable  to  perlitic 
structure  has  ever  been  detected  by  the  microscope  in  those  crystalline 
rocks  which  weather  spheroidally — there  is  nothing  in  the  microscopic 
appearance  of  basalt,  diorite,  etc.,  that  would  lead  one  to  expect  that 
such  rocks  should  exfoliate  in  successive  concentric  shells.  While, 
therefore,  we  need  have  no  doubt  as  to  the  vertical-  and  cross-jointing  of 


152  STRUCTURAL  AND  FIELD  GEOLOGY 

basalt,  etc.,  being  due  to  contraction,  the  origin  of  the  shell-like 
structure  exhibited  by  weathering  and  decomposing  rocks  is  still  an  open 
question. 

EXPANSION. — Rocks  of  all  kinds  when  subjected  to  heat  will  neces- 
sarily expand,  and  when  cooling  will  contract.  As  a  result,  they  become 
rent  and  fissured.  Excellent  examples  are  seen  in  the  Scottish  coal- 
fields. Thus  beds  of  common  coal,  subjected  to  the  heat  of  molten  rock 
erupted  in  their  immediate  neighbourhood,  have  been  converted  into 
prismatic  coke.  In  such  cases  the  coal  having  been  subjected  to 
destructive  distillation  has,  of  course,  lost  some  of  its  constituents.  Even 
siliceous  sandstones  and  argillaceous  shales  invaded  by  eruptive  rock- 
masses  often  acquire  a  rudely  columnar  structure.  But  sun-heat  is  a 
much  more  general  cause  of  expansion.  This  is  probably  effective  in  all 
latitudes,  but  naturally  enough  its  results  are  best  studied  in  dry  tropical 
and  subtropical  regions.  In  temperate  and  higher  latitudes,  the  effect  of 
insolation  is  obscured  or  entirely  concealed  by  the  much  more  energetic 
action  of  frost  and  other  epigene  agents.  In  warm  and  relatively  rainless 
regions  the  rocks  are  heated  up  during  the  day  to  a  high  temperature — 
consequently  their  superficial  portions  expand  to  such  an  extent  that 
they  often  become  detached,  and  bulge  up  from  the  underlying  rock  of 
which  they  form  a  part.  In  this  way  igneous  rocks  sometimes  acquire 
a  superficial  flaggy  structure.  When  night  falls  rapid  radiation  ensues, 
and  the  rocks  quickly  contract,  so  that  the  superficial  portions  tend  to 
break  up  more  or  less  rapidly.  In  the  case  of  fine-grained  homogeneous 
rocks,  the  highly  heated  surface  often  peals  away  in  thin  sheets  which  curl 
up,  and  are  readily  removed  by  the  wind.  The  flags  produced  by  desquama- 
tion  naturally  coincide  with  the  surface,  so  that  they  may  be  curved, 
inclined,  or  horizontal  according  as  the  rock-surface  is  rounded,  sloping, 
or  level.  Their  direction  is,  therefore,  independent  of  any  internal  rock- 
structure. 

The  cross-joints  of  granite  may,  in  like  manner,  owe  their  origin 
to  epigene  action.  All  the  phenomena  connected  with  them  seem  to 
point  to  that  conclusion.  They  are  always  approximately  parallel  with 
the  surface,  are  most  numerous  and  strongly  marked  in  the  superficial 
part  of  the  rock,  and  as  they  are  followed  downwards,  appear  at  longer 
and  longer  intervals,  becoming,  at  the  same  time,  more  interrupted  in 
their  course  and  less  conspicuous,  until  finally  they  disappear.  Further, 
it  may  be  noted  that  the  "  grain  "  or  "  rift "  of  the  granite — i.e.  the  direction 
in  which  the  rock  splits  or  breaks  most  readily,  is  parallel  with  the  cross- 
joints.  Not  only  so,  but,  like  the  latter,  it  is  most  marked  near  the 
surface,  and  gradually  dies  out  downwards.as  they  disappear.  Now  there 
is  no  apparent  petrographical  structure  to  account  for  this  "grain,"  and 
its  coincidence  with  the  cross-jointing.  The  rock  consists  throughout  of 
a  heterogeneous  pell-mell  aggregate  of  minerals.  It  is  otherwise  with 
sedimentary  rocks,  the  "grain"  or  "rift"  of  which  naturally  coincides 
with  planes  of  deposition,  just  as  the  "grain  "  of  a  schistose  rock  coincides 
usually  with  planes  of  foliation.  The  grain  of  some  crystalline  igneous 
rocks  is  due,  likewise,  to  a  roughly  parallel  arrangement  of  their  constituent 


PLATE  XXXVII. 


PLATE.  XXXVIIJ,  ,  ,    , 


[Between  pages  152  and  153. 


JOINTS  153 

minerals,  as  in  the  case  of  phonolite,  which,  owing  to  the  orientation  of 
its  dominant  minerals,  often  cleaves  more  or  less  readily  into  parallel 
slabs  and  flags.  So  again,  many  lavaform  rocks,  and  even  occasionally 
intrusive  sheets  or  sills  and  dykes,  have  a  tendency  to  split  most  readily 
in  the  direction  of  flow.  In  most  cases  the  "grain  "  is  determined  by  the 
more  or  less  obvious  parallel  arrangement  of  the  rock-ingredients — a 
rock  dividing  most  readily  when  the  orientation  is  best  developed. 
When  such  arrangement  is  very  obscure  or  not  at  all  visible,  as  in  heavy 
lavas  and  sills,  the  coincidence  of  the  "  grain  "  with  the  direction  of  flow 
seems,  nevertheless,  to  show  that  the  "grain"  is  an  original  structure. 
No  such  structures,  however,  occur  in  normal  granite — the  grain  of  which 
certainly  does  not  depend  on  the  alignment  of  its  constituents.  Neither 
does  it  bear  any  relation  to  possible  movements  of  the  original  molten 
mass,  nor  to  the  vertical  joints  or  fissures  of  retreat  produced  by  con- 
traction. It  has  apparently  been  superinduced  in  granite  by  epigene 
action — and  is  not  improbably  due  to  the  expansion  and  contraction 
induced  by  seasonal  and  secular  changes  of  temperature.  This  conclusion 
gains  support  from  the  fact  that  cross-jointing  and  coincident  "grain," 
similar  to  those  of  granite,  have  been  observed  in  other  kinds  of  eruptive 
rock.  Cross-jointing  has  even  been  superinduced  in  the  upper  parts  of 
fine-grained  homogeneous  aqueous  rocks,  where  consolidation  from  a 
state  of  igneous  fusion  is,  of  course,  excluded.  But  if  such  structures 
can  be  superinduced  by  epigene  action,  we  may  be  led  to  suspect  that  the 
exfoliating  spheroids  of  weathered  basalt,  etc.,  are  likewise  independent 
of  any  original  structure  due  to  contraction  while  the  rock  was  cooling — 
that,  in  short,  the  concentric  shells  are  solely  the  result  of  changes  of 
temperature  and  of  weathering  generally. 

CRUSTAL  MOVEMENTS. — While  it  may  be  admitted  that  the  tension 
brought  about  by  the  causes  already  considered  must  have  induced  the 
formation  of  fissures  in  many  kinds  of  rock,  there  is  yet  a  large  class  of 
joints  which  cannot  be  thus  explained.  The  regular  intersecting  systems 
of  master-joints  in  sedimentary  strata  are  suggestive  rather  of  powerful 
mechanical  stress  and  strain.  The  strata  have  been  cut  through  as 
smoothly  as  if  they  had  been  severed  by  a  knife  ;  and  this  is  true  not  only 
of  homogeneous  rocks,  such  as  limestone,  but  of  heterogeneous  aggregates 
like  conglomerate.  In  the  latter  rock  the  superinduced  division-planes 
pass  without  interruption  through  stones  and  matrix  alike,  even  although 
the  stones  may  consist  of  much  harder  and  tougher  material  than  the 
matrix  in  which  they  are  embedded.  Had  such  joints  been  the  result 
of  contraction  only,  the  stones  would  simply  have  been  pulled  out  of  the 
matrix  on  one  side  of  a  fissure  and  left  projecting  from  the  surface  of  the 
other.  The  general  opinion  is  that  joints  of  this  kind  are  the  result  of 
crustal  movements.  It  is  not  difficult  to  conceive  how  strata,  subject 
to  compression  and  tension  during  such  movements,  should  have  cracked 
and  become  fissured.  We  seem  thus  to  get  a  ready  explanation  of  the 
strike-joints,  for  it  is  along  the  axes  of  folds  that  strata  would  be  subject 
to  the  greatest  compression  and  tension.  It  seems  also  reasonable  to 
infer  that  dip-joints  might  have  originated  at  the  same  time.  Various 


154  STRUCTURAL  AND  FIELD  GEOLOGY 

interesting  experiments  by  the  famous  French  geologist,  A.  Daubree, 
tend  to  show  that  two  series  of  intersecting  joints  might  be  expected  to 
result  from  powerful  crustal  movements.  Daubree  experimented  upon 
long  rectangular  plates  composed  of  various  substances,  and  demonstrated 
that  these,  when  subjected  to  the  strain  of  torsion,  were  traversed  by  two 
sets  of  approximately  parallel  cracks,  one  system  crossing  the  other  at 
angles  of  70°  to  90°,  and  thus  closely  simulating  the  intersecting  master- 
joints  of  stratified  rocks. 

Mr  W.  O.  Crosby  has  suggested  another  explanation  of  the  normal 
intersecting  joints  so  characteristic  of  bedded  rocks.  He  thinks  that 
these  are  probably  due  to  earthquake  action.  The  fractures  produced 
by  vibratory  movements  of  the  earth's  crust  he  shows  must  be  plane, 
parallel,  intersecting,  and  normally  vertical,  thus  possessing  all  the 
characteristics  of  master-joints.  Mr  Crosby  thus  appeals  to  a  vera  causa, 
but  his  theory  does  not  exclude  that  which  would  attribute  dip-  and 
strike-joints  to  folding.  It  affords  a  better  explanation,  however,  of  the 
vertical  intersecting  joints  of  horizontal  strata,  which  can  hardly  be 
accounted  for  by  torsion.  Undisturbed  horizontal  strata,  covering  wide 
regions,  are  often  as  regularly  jointed  as  strata  which  have  been  folded. 
In  such  cases,  therefore,  we  may  suppose  the  jointing  has  most  probably 
resulted  from  the  passage  of  earthwaves  through  the  rocks,  the  alternate 
compression  and  tension  having  been  sufficient  to  produce  fissuring. 
As  there  is  possibly  no  part  of  the  earth's  crust  which  has  not  experienced 
earthquake  shocks  and  vibrations,  such  crustal  movements  may  have 
played  a  more  important  role  in  the  formation  of  joints  than  might  be 
suspected.  The  great  crustal  movements  which  resulted  in  the  buckling 
up  and  folding  of  strata  in  gigantic  mountain  chains,  must  often  have 
induced  severe  earthquakes,  caused  by  the  sudden  yielding  of  rock- 
masses  to  tension  ;  but  the  fissuring  and  shattering  due  to  the  passage 
of  such  vibrations  or  waves  of  elastic  compression  could  not  now  be 
distinguished  from  the  ordinary  effects  of  folding  and  torsion. 


CHAPTER  XI 

FAULTS   OR   DISLOCATIONS 

Normal  Faults.  Dip-faults  and  Strike-faults — their  effect  upon  Outcrops. 
Oblique  Faults.  Systems  of  Faults.  Step-faults.  Trough-  and 
Ridge-faults.  Shifting  of  Faults.  Reversed  Faults.  Transcurrent 
Faults.  Origin  of  Faults. 

HAVING  now  learned  that  rocks  of  all  kinds  are  more  or  less 
fissured,  and  that  no  small  proportion  of  the  joints  by  which 
they  are  thus  traversed  appear  to  owe  their  origin  to  crustal 
movements,  we  must  next  make  the  acquaintance  of  fissures 
of  another  kind,  known  as  Faults  or  Dislocations.  These 
are  doubtless  due  likewise  to  crustal  movements,  but  they 
differ  from  joints  in  being  not  mere  cracks  or  rents,  but 
fissures  of  displacement.  The  rocks  on  one  side  of  a  fault  are 
thus  abruptly  truncated  and  brought  against  younger  or  older 
rocks  on  the  other  side.  Three  types  of  faults  are  recognised, 
namely,  Normal  Faults  (or  Downthrows),  Reversed  Faults 
(or  Overthrusts),  and  Transcurrent  Faults  (or  Transverse 
Thrusts). 

NORMAL  FAULTS. — These  dislocations  are  rarely,  if  ever, 
quite  vertical,  although  in  natural  exposures  they  sometimes 
appear  to  be  so.  But  when  they  are  followed  downwards,  as 
in  mining  operations,  they  are  invariably  found  to  be 
inclined,  the  degree  of  inclination  varying,  it  may  be,  from 
point  to  point,  so  that  in  places  they  occasionally  show 
verticality.  The  general  inclination  of  a  fault  from  the 
vertical  is  termed  the  hade,  and  this,  in  the  case  of  normal 
faults,  is  always  in  the  direction  of  the  downthrow.  The 
degree  of  deviation  from  the  vertical  is  quite  indeterminate ; 
but,  as  a  general  rule,  the  larger  are  more  steeply  inclined 
than  the  smaller  faults.  But  to  this  rule  many  exceptions 

155 


156 


STRUCTURAL  AND  FIELD  GEOLOGY 


occur.  The  amount  of  vertical  displacement  is  known  as  the 
throw  *  of  a  fault,  and  is  measured  by  protracting  a  line  in  a 
horizontal  direction  (as  in  Fig.  33),  across  the  fault  from  the 
truncated  end  of  some  particular  bed  (a)  until  a  perpendicular 
(x — a2)  dropped  from  the  protracted  line  can  reach  the  other 
end  of  the  selected  stratum  on  the  opposite  side  of  the  fault. 
Miners  seldom  use  the  term  fault,  but  speak  of  downthrows 
or  downcasts,  and  upthrows  or  upcasts,  according  to  the 


FIG.  33. — NORMAL  FAULTS  IN  HORIZONTAL  STRATA.  € 

direction  in  which  they  are  working.  Thus  the  faults  (F1,  F2) 
shown  in  Fig.  33  would  be  described  as  downcasts  or  down- 
throws if  they  were  encountered  by  a  miner  working  in  the 
direction  from  A  to  a,  or  from  a  to  A,  but  he  would  speak  of 
them  as  upcasts  or  upthrows,  if  he  approached  them  from  the 
direction  of  C  to  a2,  or  from  B  to  a2. 

It  is  obvious  that  in  Fig.  33,  representing  faulted 
horizontal  strata,  the  amount  of  throw  is  equal  to  the 
thickness  of  the  beds  lying  between  x — a2  and  xl — a2 ; 

but  this  is  not  so  in  the 
case  of  inclined  strata.  In 
Fig.  34,  for  example,  the 
amount  of  vertical  displace- 
ment (a — x)  is  in  excess  of 
the  thickness  of  the  strata 
measured  as  in  the  pre- 


FlG.  34. 


ceding  illustration  (Fig.  33)  at  right  angles  to  the  planes  of 
bedding  (a1 — a2). 

Strata    cut    across    by   an   inclined   fault   are    not    only 
dropped  to  a  lower  level  on  the  downthrow  side,  but  the  fault 

*  The  amount  of  displacement  varies  indefinitely.  Some  faults  are 
mere  slips  of  a  few  feet  or  inches  ;  others  are  downthrows  of  several 
thousand  yards.  Between  these  extremes  all  gradations  are  met  with. 


FAULTS  157 

has  the  effect  of  producing  a  lateral  displacement  or  heave — 
the  amount  of  which  is  determined  by  the  hade  and  the 
throw.  For  example,  the  truncated  end  of  the  coal-seam  a 
(Fig.  33)  is  removed  laterally  by  F1  from  its  disconnected 
continuation  a?  by  the  distance  a — x\  but  it  is  obvious  that 
this  distance  would  be  increased  if  the  amount  of  downthrow 
were  augmented.  Again,  with  the  more  gently  inclined  fault 
(F2),  the  lateral  displacement  or  heave  is  considerably 
increased.  In  the  case  of  the  vertical  fault  (F)  there  is,  of 
course,  no  heave  or  lateral  displacement.  The  inclination 
of  faults,  therefore,  is  of  great  importance  in  a  coal-field,  for 
the  further  the  hade  deviates  from  the  vertical  the  wider  will 
be  the  extent  of  "  barren  ground  "  between  the  two  ends  of 
a  dislocated  coal-seam. 

The  faults  shown  in  the  diagrams  are  straight  lines,  but 
in  reality  faults  are  not  always  or  even  often  so  smooth  as 
they  are  here  represented  to  be.  Although  the  walls  of  a  fault 
may  be  in  close  apposition,  they  are  often  separated  either 
continuously  or  at  irregular  intervals  by  masses  of  jumbled 
and  shattered  rock-debris,  known  as  fault-rock  or  fault- 
breccia  (Plate  XXXIX.).  In  many  cases,  also,  cavities  occur 
along  a  line  of  dislocation,  and  these  are  often  filled  or  parti- 
ally filled  with  crystallised  minerals,  as  will  be  shown  more 
particularly  when  we  come  to  consider  the  phenomena  of 
lodes  or  metalliferous  veins.  The  walls  of  a  fault  and  the 
stones  in  a  fault-breccia  are  frequently  slickensided,  and 
afford  other  evidence  of  friction  and  crushing — the  rocks 
along  one  or  both  sides  being  sometimes  comminuted  or 
pulverised  for  some  inches  or  even  for  many  feet  back  from  the 
dislocation.  Phenomena  of  this  kind  occur  chiefly  in  con- 
nection with  the  more  powerful  faults — those,  namely,  which 
have  produced  the  greatest  amount  of  vertical  displacement. 
In  the  neighbourhood  of  such  large  faults  the  rocks  are  not 
only  broken  and  jumbled,  particularly  on  the  downthrow  side, 
but  on  the  same  side  strata  are  frequently  turned  up  more  or 
less  abruptly  against  the  dislocation.  On  the  high  side  of  the 
fault  there  is  usually  less  disturbance  and  distortion,  although 
the  rocks  tend  to  be  bent  over  as  if  they  had  been  dragged 
downwards  in  the  direction  of  displacement.  The  annexed 
sections  (Figs.  35,  36)  will  suffice  to  illustrate  these  appearances. 


158 


STRUCTURAL  AND  FIELD  GEOLOGY 


As  a  general  rule,  normal  faults  are  more  or  less  closely 
related  to  the  leading  or  dominant  rock-foldings  of  the 
district  in  which  they  occur.  Hence  they  can  usually  be 


FIG.  35. — NORMAL  FAULT,  NOT  ACCOMPANIED  BY  DISTORTION. 

described  as  Dip-faults  and  Strike-faults,  in  this  respect 
recalling  the  systematic  arrangement  of  the  joints  char- 
acteristic of  sedimentary  strata.  It  must  not  be  supposed, 

however,  that  the  coin- 
cidence of  faults  with 
dip  and  strike  is  always 
close.  The  most  that 
can  be  said  is  that  they 
trend  approximately  in 
those  directions.  Fre- 
quently, however,  they 
FIG.  36.— NORMAL  FAULT,  ACCOMPANIED  traverse  the  rock-folds 
BY  DISTORTION.  obliquely,  just  as  joints 

do,  and  sometimes  it  is  difficult  to  detect  any  system  or 
arrangement  among  them.  Nevertheless,  the  larger  faults 
tend,  on  the  whole,  to  coincide  more  or  less  closely  with  the 
geological  structures  referred  to — a  relation  which  can  hardly 
be  said  to  characterise  the  smaller  or  less  important  dis- 
locations. In  these  and  other  respects,  therefore,  normal 
faults  have  many  analogies  with  the  joints  of  sedimentary 
accumulations. 

Dip-faults  have  a  characteristic  effect  upon  the  outcrops 
of  rocks,  which  they  appear  to  shift.  This  is  well  seen  when 
such  a  fault  crosses  an  escarpment,  the  long  line  of  which 
is  suddenly  interrupted  and  shifted  forward  or  backward, 
according  to  the  position  from  which  we  view  it.  This 
advance  or  retreat  of  the  outcrops  along  a  line  of  dip-fault 
must  not  be  confounded  with  the  lateral  displacement  already 


FAULTS  159 

referred  to  as  the  "  heave "  of  a  fault.  It  is  the  degree  of 
inclination  or  "hade,"  and  the  amount  of  downthrow  that 
determine  the  extent  of  lateral  displacement,  or  "  heave."  A 
dip-fault,  if  vertical,  produces  no  heave ;  but  whether  vertical 
or  inclined,  it  never  fails  to  cause  an  apparent  horizontal 
shift  of  the  outcrops — either  forward  or  backward — according 
to  the  direction  of  downthrow.  This  apparent  advance  6r~" 
retreat  of  faulted  outcrops  is  greatest  when  the  angle  of  dip 
is  low,  diminishes  in  proportion  as  the  dip  augments,  and 
ceases  altogether  when  the  beds  are  vertical.  The  shifting, 
however,  is  not  the  result  of  any  horizontal  movement  along  the 
line  of  dislocation,  but  is  simply  the  effect  produced  by  denu- 
dation, as  a  glance  at  the  models  in  Fig.  37  will  show.  Let  . 
A  represent  a  block  of  strata  dipping  in  one  determinate 
direction.  A  dislocation,  we  shall  suppose,  takes  place  along 
the  interrupted  line  f  f.  In  B  we  have  the  same  model 
showing  the  vertical  displacement  effected.  A  line  dropped 
from  the  truncated  end  of  the  bed  a  to  its  disconnected  con- 
tinuation a2  is  a  perpendicular — in  other  words,  the  fault  is 
a  vertical  displacement.  Now  let  us  suppose  that  the  surface 
is  cut  even  by  denudation  along  the  line  s  s.  The  portion 
above  this  line  we  remove,  and  we  have  in  C  a  ground-plan 
of  the  new  surface  thus  produced.  It  now  becomes  evident 
that  the  horizontal  shift  is  only  apparent,  and  is  simply  the 
result  of  the  removal  of  strata  from  the  high  or  "  upcast " 
side  of  the  fault. 

When  dip-faults  traverse  'anticlinal  and  synclinal  folds, 
they  necessarily  cause  similar  apparent  horizontal  shifting  of 
outcrops.  But  as  the  outcrops  are  shifted  by  one  and  the 
same  fault  in  different  directions,  it  is  obvious  that  this  effect 
cannot  be  the  result  of  horizontal  movements.  This  is  made 
clear  by  the  models  in  Figs.  38,  39.  In  Fig.  38,  A  represents 
a  block  of  strata  arranged  in  the  form  of  a  syncline  which 
is  eventually  fractured  along  the  line  //  and  displaced  as 
shown  in  B.  When  the  strata  on  the  high  side  of  the  fault 
have  been  removed  by  denudation,  and  the  whole  area  has 
been  reduced  to  the  same  level  s  s,  the  outcrops  will  exhibit 
the  appearance  shown  in  C.  It  is  evident,  therefore,  that  a 
fault  traversing  a  syncline  has  the  effect  of  causing  a  mutual 
approach  of  the  outcrops  on  the  high  side,  and  a  correspond- 


160 


STRUCTURAL  AND  FIELD  GEOLOGY 


c 

FIG   37  —EFFECT  PRODUCED  ON          FIG.  38.— EFFECT  PRODUCED  ON  OUT- 
OUTCROPS  BY  DIP-FAULT.  CROPS  BY  DIP-FAULT  TRAVERSING 

SYNCLINAL  STRATA. 


FAULT-ROCK,  RIVER  GARRY,  AT  DALNACARDOCH,  PERTHSHIRE. 

Photo  by  H.M.  Geological  Survey. 

[To  face  page  160. 


FAULTS 


161 


FIG.    40.  —  EFFECT    PRODUCED    ON 

FIG.  39.  —  EFFECT  PRODUCED  ON  OUTCROPS  BY  STRIKE -FAULT 
OUTCROPS  BY  DIP-FAULT  TRA-  WITH  DOWNTHROW  IN  THE 
VERSING  ANTICLINAL  STRATA.  DIRECTION  OF  DIP. 

L 


162  STRUCTURAL  AND  FIELD  GEOLOGY 

ing  retreat  of  the  outcrops  in  the  opposite  direction  on  the 
low  side  of  the  dislocation.  Similar  appearances  present 
themselves  when  a  fault  cuts  across  an  anticline,  as  will  be 
seen  by  examining  the  models  in  Fig.  39.  The  anticline 
before  dislocation  is  represented  by  A,  while  B  shows  the 
dislocation  completed.  When  the  portion  lying  above  the 
line  s  s  is  removed,  we  have  the  new  surface  (C)  produced 
by  denudation,  upon  which  the  outcrops  on  the  high  side 
of  the  fault  appear  to  have  been  shifted,  but  in  opposite 
directions  to  the  apparent  shifting  produced  on  the  high 
side  of  a  fault  traversing  a  syncline.  In  the  case  of  a  faulted 
anticline  the  opposing  outcrops  on  the  "  upcast "  side  appear 
to  recede  frdm  each  other,  while  on  the  "  downcast "  side  the 
corresponding  outcrops  seem  to  have  been  brought  closer 
together,  the  appearance  of  movement  in  opposite  directions 
being,  of  course,  entirely  the  effect  of  denudation. 

Strike-faults  or,  as  they  are  often    termed,   longitudinal 
faults,   are   so   called    because    they   trend    in    the    general 
direction  of  the  strike  or  the  axes  of  the  folds  of  a  district. 
They  also  affect  the  outcrops,  but  in  a   different   way  from 
dip- faults.     They  do  not  cause  any  apparent  horizontal  shift- 
ing, and  therefore  are  not  so  easily  detected,  in  many  cases  at 
least,   as   ordinary   dip-faults.     Sometimes    their   downthrow 
is  in  the  same  direction  as  the  inclination  of  the  strata ;  at 
other  times  it  is  in  the  opposite  direction  or  against  the  dip. 
In  Fig.  40,  A  represents,  as  before,  a  block  of  strata  traversed 
by  a  strike-fault  /ft  the  vertical  displacement  being  shown 
in   B.     In   this   case   the   downthrow   of  the  fault  is  in  the 
direction  of  dip.     Removing  the  higher  portion  of  the  model 
above  the  line  s  s  in  B,  we  have  the  ground-plan  as  shown 
in  C.     Obviously,  the  effect  of  a  fault  hading  in  the  direction 
of  dip  is  to  cut  out  strata — to   carry  their   outcrops   below 
the   surface.     In   the  area  represented  by  the  model  A,  we 
have  a  considerable  succession  of  strata  numbered  consecu- 
tively, i,  2,  3,  4,  5,  6.     The  same  beds  are  shown  in  C,  but 
some  no  longer  crop  out  at  the  surface,  but  under  the  surface 
and    against  the  dislocation.     The  beds  2,  3,  and  4  are  cut 
out,  as  it  were. 

Let  us  now  take   the  case  of  a  strike-fault  which  has  a 
downthrow  against  or  in  the  opposite  direction  to  the  dip  of  the 


FAULTS  163 

strata.  The  effect  of  such  a  fault  is  precisely  the  reverse  of 
that  just  described.  Instead  of  cutting  out  strata  at  the 
surface,  it  causes  outcrops  to  be  repeated.  A  glance  at  the 
models  (Fig.  41)  will  explain  how  that  happens.  In  A  we 
have  the  strata  before  displacement  is  effected  along  the  line 
//  B  shows  the  dislocated  strata,  and  in  C  we  have  the 
effect  produced  at  the  surface  by  the  removal  of  the  block 
projecting  above  the  line  s — the  truncated  ends  of  the  beds 
i,  2,  and  3,  being  brought  up,  as  it  were,  and  caused  to  crop 
out  again,  so  as  to  present  the  deceptive  appearance  of  six 
beds  all  dipping  in  the  same  direction. 

Strike-faults  are  apt  to  be  overlooked  when  they  coincide 
more  or  less  closely  with  the  line  of  bedding.  An  observer, 
for  example,  who  should  traverse  either  of  the  areas  repre- 
sented in  the  models  C,  C  (Figs.  40,  41),  might  easily  fail  to 
detect  any  evidence  of  dislocation,  and  might  be  led  to 
suppose  that  he  had  passed  across  continuous  successions  of 
strata ;  and  thus,  in  the  one  case,  he  would  assign  too  small, 
and  in  the  other  too  great,  a  thickness  to  the  series.  It  is  rarely, 
however,  that  a  fault  follows  the  strike  continuously ;  more 
usually  it  undulates  from  one  side  to  another,  and  whenever 
it  leaves  the  strike  its  presence  is  at  once  betrayed  by  the 
more  or  less  abrupt  truncation  of  the  strata  which  it  produces. 
But  even  when  it  keeps  closely  to  the  line  of  strike,  variations 
in  the  amount  of  its  downthrow  will  nevertheless  indicate  its 
presence.  As  will  be  shown  presently,  the  throw  of  every 
fault  necessarily  varies.  Each  begins  at  zero — then  gradually 
or  more  rapidly,  as  the  line  of  fracture  is  followed,  the  down- 
throw increases  until  its  maximum  is  reached,  after  which  it 
diminishes  until  zero  is  again  reached  at  its  farther  extremity. 
In  other  words,  the  crust  is  cracked  along  a  certain  line,  and 
sinks  or  sags  on  one  side  of  that  line — the  depression  being 
greatest,  as  a  rule,  at  a  point  midway  between  the  two  ends 
of  the  rent.  The  effect  of  a  strike-fault  with  such  a  diminish- 
ing or  increasing  downthrow  is  illustrated  by  the  models 
(Fig.  42).  As  in  preceding  illustrations,  A  represents  the 
strata  before  displacement  along  the  line /"/has  been  effected. 
In  B  the  fault  has  taken  place,  the  downthrow  being  at  a 
maximum  at  .r,  but  gradually  diminishing  towards  2,  where  it 
dies  out.  When  the  portion  projecting  above  s  is  removed, 


164  STRUCTURAL  AND  FIELD  GEOLOGY 


J  3  ^ 


3  ^  t 


C  C 

FIG.    41.  — EFFECT    PRODUCED    ON  FIG.    42. —  EFFECT    PRODUCED    ON 

OUTCROPS     BY      STRIKE  -  FAULT  <^UTCROPS      BY      STRIKE  -  FAULT 

WITH      DOWNTHROW       AGAINST  WITH     A     DIMINISHING     DOWN- 

THE  DIP.  THROW. 


FAULTS 


165 


we  have  the  appearance  shown  in  C,  which  represents  the 
surface  produced  by  denudation.  It  will  be  observed  that 
upon  the  upcast  or  high  side  of  the  fault  lower  beds  succes- 
sively appear  as  the  fault  is  followed  from  s  to  x. 

Oblique  Faults. — The  effects  produced  by  normal  dip- 
faults  and  longitudinal  or  strike-faults  are  so  marked  that  the 
one  kind  of  fault  cannot  be  confounded  with  the  other.  Dip- 
faults,  however,  do  not  always  or  even  often  traverse  the 
outcrops  at  right  angles,  nor  do  longitudinal  faults  invariably 
keep  to  the  line  of  strike.  Oblique  dip-faults  and  oblique 


NORTH 


SOUTH 
FIG.  43. — EFFECT  PRODUCED  ON  OUTCROPS  BY  OBLIQUE  FAULTS. 

strike-faults  are  of  common  occurrence,  and  occasionally  the 
obliquity  of  a  fault  becomes  so  great  that  the  dislocation 
cannot  be  properly  termed  either  a  dip-fault  or  a  strike-fault. 
ki  Fig.  43,  for  example,  A  represents  a  coal-seam  dipping  due 
north,  cut  at  an  angle  of  45°  by  a  fault  having  its  downthrow 
towards  the  north-east.  It  is  obvious  that  this  fault  behaves 
partly  as  a  dip-fault,  inasmuch  as  it  produces  apparent 
horizontal  shifting  of  the  strata,  and  partly  as  a  strike-fault, 
for  it  cuts  out  a  portion  of  the  outcrop.  The  fault  represented 
in  A1  traverses  a  coal-seam  at  a  more  acute  angle,  and  thus 
ha.s  rather  the  character  of  a  strike -fault  than  a  dip-fault,  for 


166  STRUCTURAL  AND  FIELD  GEOLOGY 

it  cuts  out  a  much  larger  part  of  the  outcrop.  The  faults 
represented  in  B  and  B1  have  downthrows  in  the  opposite 
direction  to  those  shown  in  A  and  A1,  but  otherwise  they 
resemble  the  latter  in  behaving  partly  as  dip-faults  and 
partly  as  strike- faults.  It  will  be  observed,  however,  that 
the  downthrows  are  in  an  opposite  direction.  In  B,  for 
example,  a  coal-seam  is  represented,  as  before,  dipping  due 
north,  and  intersected  by  a  north-west  and  south-east  disloca- 
tion hading  towards  south-west.  The  fault  produces  the 
effect  of  a  dip-fault  by  shifting  the  outcrop,  but  since  it 
crosses  the  strike  obliquely  it  causes  a  duplication  of  the  out- 
crop between  a  and  b.  The  fault  shown  in  B1  approximates 
much  more  closely  to  the  strike,  with  the  necessary  result  that 
a  longer  stretch  of  outcrop  is  repeated.  These  diagrams 
should  be  compared  with  the  models  shown  in  Figs.  37, 
40,  and  41.  From  a  study  of  the  latter  it  becomes  evident 
that  the  effects  produced  by  the  fault  in  A1  more  closely 
resemble  those  caused  by  the  strike-fault  C  (Fig.  40),  than 
those  that  result  from  the  normal  dip-fault  C  (Fig.  37). 
Again,  the  most  notable  feature  in  the  diagram  B1  is  the 
duplication  of  the  outcrop — the  fault  having,  for  some  con- 
siderable distance,  the  effect  upon  the  outcrop  of  a  strike- 
fault  with  downthrow  against  the  dip  (Fig.  41). 

Groups  of  Faults. — Although  faults   often  occur  singly, 
more    particularly    when    their    throw    is    small — yet    they 

frequently  form  associated 
groups  or  systems.  Great 
dislocations,  for  example, 
which  often  extend  for  long 
distances,  are  rarely  un- 
accompanied by  parallel 
FIG.  44.— COMPLEX  FAULT.  faults  having  downthrows 

in   the    same    or    opposite 

directions.  Sometimes  these  parallel  dislocations  are  so 
numerous  and  occur  so  closely  together  that  it  is  often  hard 
to  say  which  is  the  main  or  principal  fault.  When  the  down- 
throw of  all  or  most  of  them  is  in  the  same  direction,  the 
result  is  practically  the  same  as  if  there  had  been  only  one 
dislocation  with  a  large  downthrow  (see  Fig.  44).  Successive 
parallel  strike- faults  having  their  downthrows  in  one  direc- 


FAULTS 


167 


tion,  or  with  some  in  one  direction  and  some  in  another, 
often  occur  in  our  coalfields,  where  they  are  known  as  step- 
faults — an  appearance  shown  in  the  diagram  (Fig.  45).  Such 


FIG.  45.—  STEP-FAULTS. 

faults,  when  their  downthrow  is  in  the  same  direction  as  the 
dip,  have  the  effect,  as  already  indicated,  of  preventing  certain 
beds  from  croping  out  at  the  surface.  On  the  other  hand, 
a  succession  of  step-faults,  each  with  its  downthrow  against 


FIG.  46.—  STEP-FAULTS  HADING  AGAINST  THE  DIP. 

the  dip,  may  cause  a  coal-seam  to  crop  out  again  and  again. 
(See  Fig.  46,  which  should  be  compared  with  the  models  in 
Fig.  41.) 

When   parallel    or    approximately    parallel    faults    hade 


FIG.  47.—  TROUGH-FAULTS  AND  RIDGE-FAULTS. 


towards  each  other,  they  produce  the  phenomena  of  trough- 
faults  (Fig.  47,  t  /).     When  they  hade  away  from  each  other 


168  STRUCTURAL  AND  FIELD  GEOLOGY 

the  converse  structure  appears  (Fig.  47,  r)  for  which  there  is 
no  name  in  English.* 

Faults  having  the  same  trend,  but  with  their  downthrows 
in  different  directions,  often  coalesce,  and  either  suddenly 
terminate  at  the  point  of  junction,  or  one  may  die  out  and 
the  other  continue  with  usually  a  diminished  throw.  But 
when  approximately  parallel  faults,  having  their  downthrows 
in  one  and  the  same  direction,  come  together,  they  almost 
invariably  continue  as  a  single  fault,  often  with  an  increased 
amount  of  downthrow. 

The  amount  of  throw  of  normal  faults  is,  as  we  have  seen, 
very  variable — not  exceeding  a  few  feet  in  some  cases,  while 
in  others  it  may  reach  thousands  of  yards,  and  thus  bring 
into  juxtaposition  rocks  of  vastly  different  ages.  The 
smaller  faults  usually  extend  for  very  short  distances,  while 
the  greater  ones  may  continue  for  hundreds  of  miles.  The 
course  of  a  large  fault  is  usually  approximately  straight  or 
gently  sinuous,  but  not  infrequently  it  is  curved.  Such 
faults  may  begin  as  a  mere  crack,  or  as  a  series  of  two  or 
more  converging  fissures,  with  little  or  no  accompanying 
rock-displacement.  But  as  it  continues,  the  throw  gradually 
augments  until  a  maximum  is  reached,  after  which  it  usually 
decreases  until  finally  the  fault  dies  out  as  it  began — in  a 
mere  crack  or  series  of  cracks.  In  many  cases,  however,  the 
throw  varies  very  irregularly  from  point  to  point. 

The  phenomena  presented  by  the  two  conjugate  systems 
of  strike-faults  and  dip-faults  which  are  so  characteristic  of 
many  regions,  lead  to  the  belief  that  these  faults  are  of  the 
same  age — that  they  came  into  existence  contemporaneously. 
This  is  suggested  by  several  considerations  to  which  refer- 
ence will  presently  be  made.  But  it  may  be  pointed  out 
here  that  strike-faults  of  contemporaneous  age  rarely  or 
never  cross  each  other ;  and  the  same  is  the  case  with 
dip-faults  belonging  to  one  and  the  same  period  of  disturb- 
ance. One  fault  may,  and  often  does,  run  into  another,  but 
it  coalesces  with  it,  and  does  not  diagonally  intersect  it. 
When  strike-faults  are  more  powerful  than  the  dip-faults  of 

*  If  one  were  required,  perhaps  ridge-faults  might  serve  ;  but  there 
are  already  so  many  superfluous  geological  terms  that  one  hesitates  to 
add  to  the  number. 


FAULTS 


169 


the  same  district,  which  is  usually  the  case,  the  latter  are 
intercepted  by  and  do  not  cross  the  former.  And  similarly, 
when  the  dip-faults  are  stronger  than  the  strike-faults,  they 
cut  these  off. 

Shifting  of  Faults.— This  latter  rule  is  so  general,  that 
when  we  find  one  fault  crossing  and  shifting  another,  we  may 
reasonably  suspect  that  the  two  faults  belong  to  different 
periods  of  disturbance. 
The  relative  age  of  inter- 
secting faults  is  at  once 
revealed  by  the  fact  that 
the  younger  dislocation 
shifts  the  older,  just  in  the 
same  way  as  any  fault  dis- 
places strata.  The  pheno- 
mena are  illustrated  by  the 
diagram  (Fig.  48),  where 

a  a  is  obviously  the  older  fault  since  it  is  displaced  by  the 
other  (b  b\ 

Displacements  of  this  kind  are  of  common  occurrence  in 
much-disturbed  regions,  and  prove  that  such  areas  have  been 


b  * 

FIG.  48.— SHIFTING  OF  ONE  FAULT 
BY  ANOTHER. 


a,  c 

FIG.  49. — INTERSECTING  FAULTS. 

The  feathered  arrows  indicate  dip  of  strata ;  the  small  arrows  show  downthrow 
side  of  faults. 

rent    and    dislocated    at   separate — often   widely   separate — 
periods.       In    countries    where  ore-bearing  veins    are    well 


170  STRUCTURAL  AND  FIELD  GEOLOGY 

developed,  these  frequently  occupy  lines  of  dislocation,  which 
intersect  at  various  angles.  In  such  regions,  therefore,  the 
direction  or  trend  of  conjugate  systems  of  faults  and  their 
relation  to  other  similar  systems  have  been  very  carefully 
studied,  and  we  now  know  that  the  dislocations  of  a  region 
may  belong  to  two,  three,  or  more  periods  of  crustal 
disturbance.  In  Fig.  49  we  have  a  model  showing  three 
lines  of  faulting,  of  which  it  is  obvious  that  the  dislocation 
a  a  must  be  the  oldest,  since  it  is  shifted  by  the  fault  b  b ; 
while  the  latter  in  its  turn  is  shifted  by  the  fault  c  c,  which, 
therefore,  must  be  the  latest  of  the  series. 

REVERSED  FAULTS. — These  faults  are  so  termed  because 
the  hade  is  not  in  the  direction  of  downthrow,  as  is  the  case 
with  normal  faults,  but  in  the  direction  of  upthrow.  Lower 
or  older  rocks  on  one  side  of  the  dislocation  have  been  thrust 
up  over  higher  or  younger  rocks  on  the  other  side.  The 
hade  of  a  reversed  fault,  especially  if  it  be  a  great  displace- 
ment, is  usually  further  inclined  from  the  vertical  than  the 
hade  of  a  correspondingly  large  normal  fault.  In  some 
cases,  indeed,  an  extensive  reversed  fault  approaches 
horizontality.  The  rocks  along  the  line  of  a  reversed  fault 
are  often  much  compressed,  crushed,  and  broken.  Not 
infrequently,  indeed,  such  a  fault  is  marked  throughout 
its  whole  extent  by  a  band  of  shattered  and  crushed  rock, 
forming  what  is  termed  a  crush-  or  friction-breccia*  The 
rocks  in  the  immediate  proximity  are  also  often  more  or 
less  metamorphosed. 

The  most  notable  reversed  faults  are  met  with  in  largest 
numbers  in  regions  of  highly  folded  and  compressed  rocks. 
They  are  of  comparatively  rare  occurrence  amongst  horizontal 
and  gently  inclined  strata.  Like  normal  faults,  they  frequently 
bear  an  obvious  relation  to  rock-folds,  and  their  phenomena 
will  be  better  understood  if  considered  in  connection  with  the 
general  question  of  the  origin  of  faults. 

TRANSCURRENT  FAULTS  OR  TRANSVERSE  THRUSTS.— 
It  has  been  explained  that  the  apparent  horizontal  shifting 
produced  by  normal  faulting  is  really  the  effect  of  denuda- 
tion. Horizontal  shifting  of  outcrops  on  the  large  scale, 

*  If  the  stones  are  subangular  or  somewhat  rounded,  the  fault-rock  is 
sometimes  termed  crush-conglomerate. 


FAULTS  171 

however,  does  actually  occur  in  certain  regions  of  highly 
folded  strata.  Faults  of  this  kind  are  steeply  inclined  or 
vertical,  and  often  extend  for  many  miles,  always  traversing 
the  strata  at  approximately  right  angles  to  the  strike.  They 
are  neither  downthrows  nor  upthrows,  movement  having 
taken  place  in  a  forward  direction,  so  that  the  walls  of  the 
thrusts  are  slickensided  horizontally.  Not  infrequently  the 
rocks  on  either  side  are  much  crushed  and  shattered,  or  the 
two  walls  may  be  separated  by  a  "  friction-breccia."  Among 
the  best-known  examples  of  such  transverse  thrusts  or  trans- 
current  faults  are  those  met  with  in  the  Alps  and  the  Jura  and 
in  the  Scottish  Highlands. 

Origin  of  Faults. — Although  we  have  yet  much  to  learn  as  to  the 
origin  of  faults,  there  are  certain  conclusions  which  seem  fairly  well 
established.  From  our  descriptions  of  the  phenomena  of  normal  faults, 
the  student  has  doubtless  gathered  that  these  displacements  are  most 
satisfactorily  explained  by  the  view  that  they  are  true  downthrows  and 
not  upthrows.  Their  somewhat  constant  relation  to  the  dominant  folds 
of  a  region  seems  to  suggest  that  they  are,  like  many  joints,  the  result 
of  torsional  strain.  We  may  suppose  that  the  rents  were  produced  or 
commenced  at  the  time  the  strata  were  being  folded.  But  folding 
implies,  of  course,  lateral  compression,  and  it  does  not  seem  likely, 
therefore,  that  the  fractured  rocks  could  subside  so  long  as  compression 
continued.  When  this  movement  had  ceased,  however,  the  bulged-up 
crust,  relieved  from  lateral  pressure,  would  tend  to  sink  again,  and 
subsidence  would  naturally  take  place  along  the  cracks  and  fissures 
which  had  already  come  into  existence.  The  whole  area  we  may  think 
of  as  being  split  up  into  a  series  of  rectangular  blocks  of  varying  size, 
each  block  defined  by  fissures,  some  of  which  would  be  inclined  in  one 
direction  and  some  in  another.  In  this  way  a  long  rectangular  block 
defined  by  two  parallel  strike-faults  inclined  towards  each  other,  would 
have  a  relatively  narrow  base  ;  while  the  adjoining  block,  defined  by 
two  parallel  strike-faults  inclined  away  from  each  other,  would  have  a 
relatively  broader  base.  Thus,  when  gravitation  came  into  play  and  the 
fractured  rock-masses  commenced  to  sink,  those  with  a  relatively  narrow 
base  (presenting  as  they  would  a  smaller  area  to  pressure)  would  tend 
to  sink  more  readily  than  the  broader  based  segments  that  adjoined 
them.  In  short,  downthrow  would  take  place  in  the  direction  of  the 
hade.  It  must  be  noted,  however,  that  some  amount  of  lateral  pressure 
would  be  exerted  by  the  several  subsiding  masses,  which  might  now  and 
again  result  in  local  distortion,  crushing,  and  fracturing,  such  as  so 
frequently  accompany  normal  faults.  This  lateral  pressure  would  also 
account  for  the  fact  that  now  and  again  the  rocks  on  both  sides  of  these 
faults  are  turned  or  bent  upwards  or  downwards  (see  Fig.  50). 

Apart,  however,  from  any  theoretical  explanation  of  normal  faults. 


172 


STRUCTURAL  AND  FIELD  GEOLOGY 


we  have  direct  evidence  to  show  that  such  faults  are  occasionally  so 
intimately  connected  with  folds  and  flexures  that  we  can  have  no  doubt 
that  they  are  contemporaneous,  and  due  to  one  and  the  same  crustal 
movement.  Again  and  again,  for  example,  large  strike-faults,  when  traced 
continuously,  have  been  found  to  die  out  in  a  flexure.  In  the  case  of 
monoclinal  flexures  it  is  not  hard  to  see  how  that  should  be.  Strain 
or  tension  must  be  set  up  along  the  margin  of  a  sinking  area.  If 


FIG.  50.— PARALLEL  FAULTS  WITH  DISTORTED  STRATA  BETWEEN. 

subsidence  should  take  place  within  a  region  built  up  of  horizontal  strata, 
the  horizontal  position  of  the  rocks  along  the  boundary  or  margin  of  the 
sinking  area  will  be  interfered  with.  The  pull  or  drag  of  the  descending 
mass  will  cause  the  strata  of  the  adjacent  stable  area  either  to  bend  over 
or  to  snap  across.  Should  the  movement  be  slow  and  protracted,  the 
rocks  will  probably  at  first  yield  by  bending.  They  will  be  turned 
downwards  and  compressed,  it  may  be,  by  stretching.  But  should  the 
movement  continue,  they  must  eventually  give  way,  and  a  fold  will  thus 
be  replaced  by  a  fracture.  Towards  one  or  both  ends,  therefore,  we 
should  expect  such  a  fault  to  die  out  into  a  simple  flexure  or  monocline 
(see  Fig.  51). 

Although  faults  of  the  kind  described  may  be  considered  the  result 
of  direct  subsidence,  it  is  obvious  that  they  might  equally  well  have 
resulted  from  movements  of  elevation.  During  the  slow  uplifting  of  a 
broad  plateau,  tension  will  come  into  play  along  the  margin  of  the 
rising  area.  Flexures  will  then  be  formed,  and  these  will  eventually  be 
replaced  by  rents  and  dislocations.  The  resulting  structure  would  thus 
be  the  same  as  if  folding  and  faulting  had  been  caused  by  a  movement 
of  subsidence.  Thus,  in  Fig.  51,  the  fault  /  might  have  been  caused 
either  by  the  subsidence  of  the  strata  at  x,  or  by  the  upheaval  of  the 
strata  at  a,  But  as  the  dominant  movement  of  the  earth's  crust  must 


FAULTS 


173 


be  one  of  subsidence,  it  is  preferable  to  consider  all  normal  faults  as 
downthrows  rather  than  upthrows.  Nevertheless,  our  knowledge  of  the 
nature  of  crustal  movements  is  not  so  complete  that  we  can  deny  the 
possibility,  or  even  the  probability,  that  normal  faults  may  now  and 
again  have  come  into  existence  during  movements  of  upheaval. 


FIG.  51.— MONOCLINAL  FLEXURE  PASSING  INTO  A  NORMAL  STRIKE-FAULT, 
VIEWED  IN  OPPOSITE  DIRECTIONS. 

(After  Chamberlin  and  Salisbury.) 

Reversed  faults  do  not  often  occur  in  regions  where  the  rocks  show 
little  trace  of  disturbance.  This  seems  to  be  due  to  the  simple  fact 
that  they  are  the  result  of  lateral  pressure  or  compression,  so  that  they 
are  best  developed  and  of  most  common  occurrence  amongst  highly 
folded  and  distorted  rocks.  Now  and  again,  however,  where  monoclinal 
flexures  have  obviously  resulted  from  horizontal  movements,  the  flexures 
have  yielded  and  given  place  to  reversed  faults  (see  Fig.  52).  In 


FIG.  52.— REVERSED  FAULT  REPLACING  MONOCLINAL  FLEXURE. 

other  cases  gently  inclined  strata,  when  subjected  to  lateral  pressure, 
have,  instead  of  first  folding,  at  once  yielded  to  the  strain  by  snapping 
obliquely — and  one  portion  of  the  severed  mass  has  been  pushed  bodily 


174  STRUCTURAL  AND  FIELD  GEOLOGY 

over  the  other.     But  faults   of  this  kind,  occurring   in  gently  inclined 
strata,  are  usually  on  a  small  scale,  and  merely  of  local  importance. 

One  of  the  commonest  kinds  of  reversed  fault  is  that  known  as  an 
overthrust.  All  folds,  as  we  have  seen,  are  the  result  of  horizontal  move- 
ment or  tangential  pressure.  When  this  is  excessive,  folds  are  rendered 
more  and  more  unsymmetrical,  the  middle  limb  of  each  fold  becoming 
thinner  and  thinner  as  the  rock  is  drawn  out  in  the  direction  of  move- 
ment. With  continued  pressure  the  limb  at  length  yields,  and  the  highly 
inclined  or  recumbent  anticline  is  pushed  forward — in  short,  the  fold  is 
dislocated  and  a  reversed  fault  comes  into  existence.  All  gradations  of 
such  overthrusts  may  be  studied  in  most  regions  of  highly  folded  and 
contorted  strata  (see  Fig.  53).  So  overpowering  has  been  the  horizontal 


FIG.  53. — ORIGIN  OF  REVERSED  FAULTS  IN  HIGHLY  FOLDED  ROCKS. 

movement  in  some  cases,  that  masses  of  rock  thousands  of  feet  in  thick- 
ness have  been  buckled  up  and  sheared.  In  other  cases,  however, 
great  reversed  faults  have  been  produced  without  much  preliminary 
buckling  or  folding  of  the  rocks.  Many  remarkable  examples  of  this 
kind  occur  in  the  north-west  of  Scotland.  In  that  region  sheet  after 
sheet  of  rock  has  been  successively  sliced  off  and  driven  forward,  some- 
times for  ten  miles  or  more,  so  that  the  oldest  rocks  often  overlie  the 
youngest  rocks  (see  Plates  XL.,  XLL). 

Plate  XL.  is  a  view  of  Sgurr  Ruadh,  a  mountain  in  Ross-shire.  It  shows 
the  north  face  of  the  mountain  along  which  the  following  series  of  bedded 
rocks  crop  out : — a,  Torridon  Sandstones  ;  <£,  Basal  Quartzite  ;  c,  Pipe-rock. 
The  white  lines  are  thrust-planes  which  traverse  the  hill-face  in  the  same 
direction  as  the  outcrops.  Plate  XLL,  for  which  I  am  indebted  to  my  old 
colleague  and  friend,  Dr  Peach,  is  a  section  taken  obliquely  across  the 
mountain,  and  shows  the  general  structure  of  the  ground.  From  the 
base  of  the  mountain  up  to  the  first  thrust-plane  the  strata  occur  in  their 
true  order  ;  thereafter,  it  will  be  seen  they  are  inverted,  and  have  been 
driven  forward.  The  two  thrust-planes  which  appear  in  section  on  the 
hill-face  are  branches  of  one  and  the  same  overthrust,  as  is  shown  in  the 
section.  The  thrust-planes  of  the  north-west  Highlands  are  inclined  at 
various  angles,  but  the  larger  ones  usually  deviate  most  from  the  vertical. 
Not  infrequently,  indeed,  they  are  almost  horizontal.  In  the  general 


PLATE  XLI. 


FAULTS  175 

denudation  of  the  country  these  thrust-planes  have  now  and  again  given 
rise  to  marked  surface-features.  As  lines  of  weakness,  for  example,  they 
are  sometimes  followed  by  streams— a  good  instance  being  shown  in 
Plate  XLII, 

Transverse  thrusts  are  obviously  due  to  the  same  cause  as  overthrusts 
— tangential  pressure, — and  may  be  looked  upon  as  contemporaneous 
with  the  folds  and  overthrusts  of  the  region  in  which  they  occur.  When 
the  strata  of  a  growing  mountain  chain  were  being  compressed  and 
pushed  forward  in  some  particular  direction,  there  might  well  be  in- 
equalities in  the  crustal  creep.  Some  portions  of  the  moving  mass  would 
advance  more  rapidly  than  others,  and  thus  cause  strain  and  tension,  to 
which  the  crust  would  necessarily  yield  in  the  direction  of  the  horizontal 
movement.  The  vertical  fissures  and  fractures  or  glide-planes  thus  formed 
would,  therefore,  traverse  the  dominant  folds  and  overthrusts  of  the  chain 
at  right  angles. 

The  general  conclusion,  then,  to  which  the  evidence  leads  is  simply 
this,  that  faults  are  usually  connected  with  folds,  or,  at  all  events,  with 
horizontal  movements  of  the  crust.  When  strata  are  sufficiently  com- 
pressed, they  usually  double  up,  and  with  continued  pressure  eventually 
yield,  and  overthrusts  take  place.  In  some  cases,  however,  in  place  of 
becoming  folded,  they  at  once  shear,  and  approximately  horizontal  or 
steeper  overthrusts  come  into  existence.  Again,  yielding  takes  place, 
and  transcurrent  faults  make  their  appearance  between  contiguous 
masses  which  are  moving  horizontally  at  different  rates  in  the  same 
direction. 

The  overthrusts  of  a  highly  disturbed  region  are  often  cut  across  by 
a  series  of  normal  faults,  and  such  phenomena  seem  to  suggest  that, 
while  overthrusts  and  transverse  thrusts  are  the  result  of  horizontal 
movements,  the  normal  faults  in  question  may  have  come  into  existence 
when  the  tangential  pressure  was  relieved.  During  a  great  horizontal 
movement,  the  rocks  are  not  only  subjected  to  enormous  compression,  but 
the  crust  is  bulged  up — in  other  words,  mountains  of  elevation  are  formed. 
When  the  forward  movement  ceases  and  pressure  is  relieved,  the  pro- 
tuberant rock-masses  will  tend  to  settle  down  to  some  extent  along  the 
rents  and  fissures  opened  during  the  elevatory  process,  and  normal  faults 
will  thus  be  produced.  In  the  case  of  normal  faults  traversing  horizontal 
and  gently  inclined  strata,  all  we  can  say  is  that,  like  joints,  they  probably 
owe  their  origin  to  torsion,  and  since  they  are  so  commonly  related  to 
strike  and  dip,  it  seems  highly  probable  that  they  also  are  the  direct 
result  of  crustal  folding.  But  we  must  not  forget  that  many  normal  faults 
have  been  formed  during  movements  of  direct  subsidence — which  may 
not  necessarily  have  been  connected  with  any  horizontal  movements  of 
the  crust.  Not  a  few  extensive  basin-  or  trough-shaped  depressions  are 
bounded  by  normal  faults,  and  are  obviously  the  result  of  direct  subsidence 
or  collapse  of  the  crust. 

Crustal  deformation  would  appear  usually  to  have  been  slowly  effected. 
Under  sufficient  pressure  solids  can  be  compelled  to  flow,  and  hard  rocks 
may  be  bent  without  fracturing,  provided  the  pressure  be  applied  gradually. 


176  STRUCTURAL  AND  FIELD  GEOLOGY 

It  is  impossible  to  believe  that  the  folded  strata  seen  in  a  mountain  range 
could  have  been  so  sharply  curved  and  plicated  without  fracture,  unless 
as  the  result  of  powerful  pressure  slowly  applied.  As  regards  dislocations, 
there  is  no  evidence  to  show  that  great  rock-displacements  have  been 
more  rapidly  effected  than  conspicuous  rock-folds.  We  need  not  go  so 
far,  however,  as  to  infer  that  all  faults  have  been  slow  creeps.  Some 
dislocations  may  have  been  more  or  less  rapidly  effected.  That  crustal 
deformation,  as  a  rule,  is  really  a  protracted  process,  is  strongly  suggested 
by  the  fact  that  folds  and  faults  have  come  into  existence  in  certain 
regions  without  disturbing  their  drainage  systems.  The  Colorado  Plateau, 
for  example,  has  been  split  across  by  well-marked  normal  faults,  some 
of  which  have  a  downthrow  of  several  thousand  feet,  and  can  be  followed 
for  hundreds  of  miles.  The  same  region  also  shows  some  notable 
folds  and  flexures,  both  faults  and  flexures  being  of  relatively  recent 
geological  age.  Yet  none  of  these  crustal  deformations  has  disturbed 
the  course  of  the  River  Colorado,  which  was  certainly  in  existence  long 
before  they  had  been  effected.  It  is  obvious,  therefore,  that  flexuring 
and  faulting  must  have  taken  place  so  gradually,  that  the  river  was  able 
to  saw  its  way  across  the  inequalities  as  fast  as  these  appeared.  Similar 
evidence  to  the  same  effect  is  supplied  by  the  river-valleys  of  the 
Himalayas.  It  is  well  known  that  the  sub- Himalayan  ranges  are  com- 
posed of  materials  derived  by  existing  rivers  from  the  central  ranges  of 
the  great  chain.  The  materials  referred  to  form  massive  accumulations 
which  have  been  disturbed  and  upheaved,  the  axes  of  the  flexures  crossing 
the  river-valleys  more  or  less  directly.  Here,  then,  it  is  evident  that 
"  the  rivers  are  older  than  the  hills  they  traverse,  and  that  the  gorges 
have  been  gradually  cut  through  the  hills  as  they  were  slowly  upheaved." 
Yet  another  example  may  be  cited  from  our  own  Continent.  Deep 
borings  have  shown  that  the  Pleistocene  deposits  in  the  valley  of  the 
Rhine  in  Hesse  occupy  a  profound  hollow,  surrounded  on  all  sides  by 
older  rocks,  the  bottom  of  the  basin  being  270  feet  deeper  than  the 
lowest  part  of  its  rim  at  Bingen.  These  deposits,  however,  are  not 
lacustrine,  but  fluviatile. .  Hence  we  must  infer  that  fluviatile  deposition 
has  kept  pace  with  the  crustal  movement.  As  the  bottom  of  the  Rhine 
valley  has  slowly  subsided,  the  river  has  flowed  on  without  interruption, 
continuously  filling  up  the  gradually  deepening  basin  with  its  sediment. 


STREAM  RUNNING  IN  LINE  OF  THRUST-PLANE,  ALLT  MOR,  KISHORN,  ROSS-SHIRE. 

Photo  by  H.M.  Geological  Survey. 

[To  face  page  176. 


CHAPTER  XII 

STRUCTURES   RESULTING   FROM   DENUDATION 
Outliers  and  Inliers.     Unconformity.     Overlap. 

IN  preceding  chapters,  frequent  reference  has  been  made  to 
denudation  in  explanation  of  certain  constantly  occurring 
phenomena.  For  example,  it  was  necessary  to  point  out 
that  plutonic  rocks  are  now  visible  at  the  surface  simply 
because  they  have  been  bared  or  denuded  by  long-continued 
epigene  action.  The  student  who  has  followed  so  far  must 
also  have  realised  that  the  very  existence  of  sedimentary 
rocks  is  evidence  of  denudation — for  every  bed  of  the  kind 
has  been  derived  from  the  breaking-up  and  disintegration  of 
some  pre-existing  rock  or  rocks.  Denudation  and  sedimenta- 
tion, in  short,  go  hand  in  hand.  We  cannot  have  the  one 
without  the  other.  In  the  sequel,  we  shall  discuss  the  subject 
of  denudation,  with  special  reference  to  the  surface-features 
of  the  land  ;  but,  for  the  present,  attention  must  be  confined  to 
certain  fock-structures  which  may  be  said  to  owe  their  origin 
to  denudation. 

Outliers  and  Inliers. — All  land-surfaces  are  necessarily 
subject  to  degradation,  and  the  marks  of  such  degradation  or 
wearing-away  are  necessarily  most  conspicuous  in  regions 
which  have  been  longest  exposed  to  epigene  action.  Vast 
masses  of  rock  have  been  gradually  removed  from  such 
regions,  so  that  many  formations  which  formerly  extended 
continuously  over  wide  areas  have  been  greatly  reduced. 
Sometimes,  indeed,  they  are  now  represented  by  only  a  few 
interrupted  sheets  and  isolated  patches.  Such  is  the  origin 
of  outliers.  An  Outlier,  then,  is  simply  a  relic  of  some  more 
or  less  extensive  bed,  or  series  of  beds,  and  may  be  shortly 
177  M 


178  STRUCTURAL  AND  FIELD  GEOLOGY 

defined  as  a  detached  area  of  rock,  surrounded  on  all  sides 
by  rocks  which  are  geologically  older  than  itself.  Such  being 
the  case,  outliers  very  often  appear  capping  hills  and  ridges. 
They  occur  amongst  all  kinds  of  rocks,  no  matter  how  these 
may  be  arranged — whether  they  be  horizontal,  inclined,  or 
highly  flexed  and  folded.  Examples  are  met  with  almost 
everywhere  throughout  these  islands.  They  often  appear 
scattered  along  the  front  of  prominent  escarpments,  of  which 
they  are  really  the  outposts,  as  may  be  seen  in  Fig.  54,  which 


FIG.  54. — ESCARPMENT,  E,  AND  OUTLIER,  O. 

represents  diagrammatically  the  outliers  and  escarpments  of 
the  Jurassic  and  Cretaceous  strata  of  Central  England.  These 
outliers  are  obviously  detached  portions  of  the  more  durable 
strata,  of  which  the  escarpments  are  composed,  and  have  been 
left  behind,  so  to  speak,  during  the  slow  retreat  of  the  latter 
under  the  influence  of  denudation.  Sometimes  outliers  owe 
their  preservation  not  so  much  to  the  durability  of  their  rocks, 
as  to  their  relatively  strong  geological  structure.  Hence,  not 
infrequently,  outliers  occur  in  synclinally  arranged  strata 
(Fig.  55,  O).  The  rocks  of  which  an  outlier  is  composed  may 
all  belong  to  one  and  the  same  series,  or  the  upper  portion 
may  rest  discordantly  upon  the  lower — showing  that  they 
belong  to  very  different  geological  horizons  (Fig.  55,  O1). 

Although  outliers  usually  occur  on  high-lying  ground  as 
the  direct  result  of  denudation,  yet  they  occasionally  owe  their 
existence  to  faults,  and  in  such  cases  they  may  appear  either 
on  heights  or  in  depressions.  Trough-faults,  for  example, 
necessarily  bring  down  younger  beds  against  older  formations, 
and  thus  detached  portions  of  strata  are  preserved — the  rocks 
with  which  they  were  formerly  connected  having  been  entirely 
removed  from  the  immediate  neighbourhood. 

An  Inlier  is  the  converse  of  an  outlier,  and  consists  of 
rocks  which  are  surrounded  on  all  sides  by  rocks  which  are 
geologically  younger.  The  rocks  of  an  inlier  may  belong  to 


STRUCTURES  DUE  TO  DENUDATION 


179 


the  same  geological  series  as  those  by  which  it  is  surrounded, 
or  they  may  be  overlaid  discordantly  by  the  latter  (Fig. 
55,  I,  I1).  As  an  inlier  is  the  result  of  denudation,  and  due 


FIG.  55.— OUTLIERS  (O)  AND  INLISRS  (I)  IN  CONFORMABLE  AND 
UNCONFORMABLE  STRATA. 

simply  to  the  partial  removal  of  overlying  rocks,  the  structure 
is  most  frequently  encountered  in  valleys  and  other  depres- 
sions. Now  and  again,  however,  an  inlier  appears  along  the 
back  of  a  denuded  anticline,  as  in  the  case  of  the  Carboni- 
ferous Limestone  of  Roman  Camp  Hill,  near  Edinburgh 


FIG.  56.— SUMMIT  OF  AN  ANTICLINE  FORMING  AN  INLIER. 

(Fig.  56,  I).  Faulting  also  sometimes  accounts  for  the  pres- 
ence of  an  inlier  forming  elevated  ground.  For  example,  we 
occasionally  encounter  hills  composed  of  ancient  rocks  rising 
more  or  less  abruptly  out  of  plains  or  plateaus  consisting  of 
younger  formations.  This  is  the  structure  of  the  "  Horst 


FIG.  57. — INLIER  RESULTING  FROM  FAULTING. 

<t,  schistose  rocks  ;  b,  b,  younger  sedimentary  strata ;  /,  /,  faults; 
o,  outlier  of  b ;  I,  inlier  of  a. 

mountains  "  of  German  geologises,  the  general  characters   of 
which  are  shown  in  Fig.  57.     In  this  case  it  is  obvious  that 


180 


STRUCTURAL  AND  FIELD  GEOLOGY 


the  inlier  represents  a  higher  crustal  level — the  Horst  owes 
its  existence  as  such  to  the  faults  or  dislocations  by  which  it 
is  bounded.  An  old  plateau-land  has  been  fractured — the 
tracts  surrounding  the  inlier  having  broken  away  from  it, 
and  dropped  to  a  lower  position. 

Conformity  and  Unconformity. — When  one  series  of 
strata  has  been  laid  down  upon  the  undisturbed  and  un- 
denuded  surface  of  another  series,  so  as  to  form  a  continuous 
succession,  the  beds  are  said  to  be  conformable — the  structure 
being  known  as  conformability  or  conformity.  In  a  true 


FIG.  58.— MARKED  UNCONFORMITY  IN  HORIZONTAL  STRATA. 

conformity,  therefore,  each  successive  bed  rests  regularly 
upon  its  predecessor.  When,  on  the  other  hand,  one  set  of 
beds  has  been  deposited  upon  the  worn  or  denuded  surface 
of  another  and  older  series,  we  have  what  is  termed  an 
unconformity  or  unconformability,  and  the  two  sets  of  beds 
are  said  to  be  unconformable[:with  each  other.  Unconformity 


FIG.  59.— INCIDENTAL  EVIDENCE  OF  UNCONFORMITY  IN  HORIZONTAL 

STRATA. 

x,  x,  unconformable  junction  ;  d,  d,  dykes  ;  /,  fault. 

sometimes  occurs  without  any  change  in  the  relative  position 
of  the  younger  and  older  strata.  Both  may  be  horizontal,  or 
may  dip  at  the  same  angle  and  in  one  and  the  same  direction 
(Figs.  58,  59).  In  such  cases  the  lower  series  will  usually 
afford  evidence  of  having  been  more  or  less  denuded  before 
the  deposition  of  the  overlying  series  had  commenced. 
Occasionally,  however,  such  evidence  of  a  physical  break  or 


STRUCTURES  DUE  TO  DENUDATION  181 

interruption  of  sedimentation  is  hard  to  detect  in  individual 
exposures  or  sections.  But  when  the  strata  are  traced  over 
some  considerable  area  the  actual  discordance  will  be  shown 
by  the  manner  in  which  the  upper  gradually  steals  over  the 
outcrops  of  the  lower  series.  In  cases  of  this  kind  the 
presence  of  an  unconformity  is  often  indicated  by  the  occur- 
rence of  rolled  or  angular  fragments  of  the  lower  rocks 
enclosed  in  the  lowest  bed  or  beds  of  the  upper  series. 
Indeed,  conglomerate  and  grit  frequently  appear  along  every 
kind  of  unconformable  junction.  Again,  the  presence  of 
dykes  of  igneous  rock  and  dislocations  in  the  lower  series 
and  their  absence  from  the  overlying  beds — dykes  and  faults 
terminating  abruptly  at  a  given  line  of  junction — would  be 
convincing  evidence  of  a  "break  in  the  succession"  (see 
Fig.  59).  For  it  is  highly  improbable  that  two  or  more  dykes 
should  terminate  upwards  at  exactly  the  same  level ;  while 
we  may  feel  assured  that  if  the  dislocations  visible  in  the 
lower  beds  do  not  extend  into  the  overlying  strata,  the  latter 
must  be  resting  upon  a  denuded  surface. 

When  none  of  these  incidental  proofs  of  unconformity  is  present,  the 
evidence  of  fossils  may  yet  be  available.  The  assemblage  of  fossils 
occurring  in  the  lower  beds  may  be  more  or  less  strongly  contrasted 
with  that  of  the  overlying  series,  and  so  lead  to  the  conviction  that  the 
appearance  of  conformity  is  deceptive.  Such  an  abrupt  break  in  the 
continuity  of  life-forms  is  termed  by  palaeontologists  a  "break  in 
succession,"  and  indicates  a  gap  or  imperfection  in  the  record,  which 
usually  implies  a  long  lapse  of  time.  We  must  allow  for  the  gradual 
extinction  of  the  old  forms  of  life  occurring  in  the  lower  beds,  and  for 
the  gradual  introduction  of  the  different  series  of  types  which  appear  in 
the  upper  beds.  In  short,  the  apparent  conformity  in  such  a  case  is 
deceptive — it  is  in  reality  an  unconformity. 

Usually,  however,  unconformity  is  marked  by  some  dis- 
cordance of  inclination — one  set  of  beds  often  resting  upon 
the  upturned  and  denuded  edges  of  an  older  series  (see  Figs. 
60,  61).  Thus  the  lower  beds  may  be  inclined  and  the 
overlying  strata  horizontal ;  or  both  may  be  inclined  in  the 
same  or  in  different  directions.  Strongly  marked  discord- 
ances of  this  kind  are  not  hard  to  trace,  even  when  there  is 
no  section  to  show  the  actual  junction  of  the  two  sets  of 
strata. 

Conformity,  as  a  rule,  indicates  more  or   less  continuous 


182 


STRUCTURAL  AND  FIELD  GEOLOGY 


sedimentation  or  accumulation — a  persistence,  upon  the 
whole,  of  the  same  physical  conditions.  It  does  not, 
however,  prove  that  the  area  of  deposition  was  stable.  On 

the  contrary,  a  thick 
series  of  conformable 
strata  of  shallow-water 
origin  could  only  have 
been  accumulated  during 
gradual  subsidence  of 
FIG.  60.— STRONG  UNCONFORMITY.  the  area.  The  evidence 

supplied     by    palaeonto- 

logical  "breaks  in  the  succession,"  further  shows  that  the 
accumulation  of  apparently  conformable  strata  has  sometimes 
been  interrupted  for  prolonged  intervals  of  time.  But  these 


FIG.  61. — Two  UNCONFORMITIES. 

are  really  cases  of  unconformity,  and  do  not  invalidate  the 
general  rule  that  true  conformity  indicates  a  persistence  of 
the  same  physical  conditions. 

It  must  be  remembered,  however,  that  conformable  strata 
have  not  necessarily  accumulated  during  one  continuous 
movement  of  subsidence.  As  already  pointed  out  (see  p.  I  ip), 
both  downward  and  upward  crustal  movements  may  take 
place  during  the  deposition  of  a  long  series  of  perfectly 
conformable  strata,  and  these  changes  may  sometimes  lead 
to  longer  or  shorter  pauses  in  the  process  of  accumulation. 
While,  therefore,  it  holds  generally  true,  that  conformity  is 
the  result  of  more  or  less  continuous  sedimentation,  we  must 
allow  for  such  interruptions  of  the  process  as  those  discussed 
in  Chapter  VII. 

Unconformity,  on  the  other  hand,  obviously  implies  an 
interruption  of  sedimentation  or  accumulation,  and  the 
supervening  of  erosion  and  denudation — or,  in  other  words, 
a  change  of  physical  conditions.  In  short,  unconformity 


STRUCTURES  DUE  TO  DENUDATION  183 

points  usually  to  the  following  succession  of  changes :  (i)  a 
period  of  accumulation — either  lacustrine  or  marine ;  (2)  a 
crustal  movement  resulting  in  the  conversion  of  the  area  of 
sedimentation  into  dry  land ;  (3)  a  more  or  less  prolonged 
period  of  erosion,  during  which  the  land  surface  is  denuded ; 
(4)  renewed  subsidence  and  deposition  of  younger  accumula- 
tions over  the  worn  and  irregular  surface  of  the  now  drowned 
land  ;  (5)  final  re-elevation  of  the  area. 

Overlap. — When  the  upper  beds  of  a  conformable  series 
extend  over  a  wider  area  than  the  lower  beds  of  the  same 
series,  we  have  the  structure  known  as  overlap.  The 
structure  indicates  subsidence  accompanied  by  sedimentation 
over  a  gradually  extending  area,  and  overlap  is  therefore 
often  well  displayed  in  cases  of  marked  unconformity.  In 
the  accompanying  section  (Fig.  62),  for  example,  the  older 


FIG.  62.— UNCONFORMITY  AND  OVERLAP. 

rocks,  <7,  have  been  much  eroded,  so  that  when  submerged 
they  formed  a  very  irregular  sea-floor.  The  hollows  being 
gradually  filled  with  sediment,  b,  it  is  obvious  that  the  upper 
must  overlap  the  lower  beds — each  stratum  extending  over 
a  wider  area  than  its  predecessors.  But  overlapping  must 
take  place  in  every  case  of  the  gradual  subsidence  of  land, 
whether  the  surface  of  the  sinking  area  be  irregular  or 
relatively  smooth.  As  the  land  sinks,  shore-deposits  become 
overlapped  by  infra-littoral  deposits,  and  these  last  by  the 
accumulations  of  deeper  water. 

Overlap  is  a  structure  which  is  not  only  interesting  and 
instructive  to  the  geologist,  but  it  has  also  an  obvious 
practical  bearing.  In  questions  of  boring  for  bedded 
minerals  it  is  often  of  the  utmost  importance,  and  failure 
to  recognise  the  structure  has  led  to  disappointment  and 
loss  which  might  have  been  avoided.  Similarly,  in  questions 
of  water-supply,  the  possible  occurrence  of  overlap  and  uncon- 
formity cannot  be  safely  disregarded, 


CHAPTER  XIII 

ERUPTIVE   ROCKS:    MODE   OF   THEIR  OCCURRENCE 

Intrusive  Eruptive  Rocks.  Plutonic  or  Abyssal  and  Hypabyssal  Rocks 
— their  General  Petrographical  Characters.  Batholiths — Granite  as 
a  type  ;  phenomena  along  line  of  Junction  with  Contiguous  Rocks  ; 
Xenoliths  ;  speculations  as  to  Assimilation  of  Rocks  by  Granite,  etc. 
Laccoliths  of  North  America.  Sills  or  Intrusive  Sheets  appear  to 
be  much-denuded  Laccoliths.  Necks  or  Pipes  of  Eruption — their 
General  Phenomena. 

IGNEOUS  rocks  have  either  been  extruded  at  the  surface, 
as  in  the  case  of  volcanic  eruptions,  or  they  have  cooled  and 
consolidated  below  ground,  and  are  now  exposed  to  the  light 
of  day  owing  to  the  removal  by  denudation  of  the  rock-masses 
underneath  which  they  formerly  lay  concealed.  We  have 
thus  two  types  of  eruptive  rocks,  namely,  effusive  and  intrusive, 
the  latter  of  which  is  most  conveniently  described  first. 

INTRUSIVE    ERUPTIVE    ROCKS 

These  rocks  are  sometimes  termed  subsequent,  with 
reference  to  the  fact  that  they  are  of  subsequent  origin  to 
the  rock-masses  with  which  they  are  associated.  Two  groups 
of  intrusive  rocks  are  recognised,  namely,  Plutonic  or 
Abyssal  and  Hypabyssal  rocks,  the  former  having  consoli- 
dated at  great  depths  in  the  crust,  while  the  latter  are  of 
less  deep-seated  origin.  It  must  be  admitted,  however,  that 
no  clear  line  of  demarcation  separates  these  two  groups — 
the  one  type  of  rock  passing  into  the  other.  Nevertheless, 
the  extremes  of  the  two  series  are  more  or  less  strongly 
differentiated  by  their  petrographical  characters,  and  also  to 
some  extent  by  the  mode  of  their  occurrence. 

The   Plutonic   or    more   deeply  seated   rocks   are   never 

184 


STRUCTURE  OF  ERUPTIVE  ROCKS  185 

vesicular  or  slaggy,  and  contain  no  glass.  Moreover,  they 
are  usually — not  always — rather  coarsely  crystalline,  and 
generally  granitoid  in  texture.  Their  constituent  minerals 
are  often  crowded  with  fluid-cavities,  while  glass-  and  stone- 
cavities  are  wanting.  The  Hypabyssal  or  less  deeply  seated 
rocks  occasionally  exhibit  all  these  characters,  but  they  also 
not  infrequently  contain  sporadic  areas  of  vesicles,  and  even, 
it  may  be,  some  residual  glassy  base  or  devitrified  matter. 
Although  often  coarsely  crystalline,  they  commonly  assume 
a  fine-grained  and  sometimes  a  compact  texture.  Hypabyssal 
rocks  thus  frequently  have  a  strong  resemblance  to  effusive 
or  lavaform  rocks,  from  which,  indeed,  it  is  often  quite 
impossible  to  distinguish  them  in  mere  hand-specimens.  The 
contrast  between  these  two  types  is  consequently  much  less 
marked  than  that  between  plutonic  rocks  and  true  lavas. 
Even  in  hand-specimens  a  truly  plutonic  or  abyssal  rock  can 
rarely  or  never  be  confounded  with  one  which  has  flowed  out 
at  the  surface  and  consolidated  under  the  ordinary  pressure 
of  the  atmosphere. 

The  true  character  of  an  igneous  rock,  however,  can  only 
be  satisfactorily  determined  by  studying  it  in  the  field,  and 
observing  its  relation  to  the  other  rock-masses  amongst  which 
it  occurs.  Usually  it  is  not  difficult  to  recognise  an  intrusive 
rock,  since  its  junction  with  surrounding  rock-masses  is 
generally  more  or  less  irregular  or  discordant.  Many 
observations  in  all  parts  of  the  world  have  shown  that 
molten  matter  invading  the  crust  from  below  has  usually 
followed  what  may  be  considered  lines  of  weakness.  That 
crust,  as  we  have  now  learned,  is  by  no  means  homogeneous, 
but  built  up  of  a  great  variety  of  rocks  arranged  in  many 
different  ways,  and  traversed  by  an  infinity  of  regular  and 
irregular  cracks,  fissures,  rents,  and  dislocations,  many  of 
which  are  vertical  or  approximately  so,  while  others  are 
inclined  at  all  angles.  All  such  original  and  superinduced 
planes  of  division — whether  planes  of  bedding,  cleavage,  or 
foliation,  whether  unconformable  junctions,  joints,  or  faults — 
are  lines  of  weakness  along  which  molten  matter  has  from 
time  to  time  found  more  or  less  ready  passage  to  the  surface. 
Further,  it  may  be  noted  that  molten  matter  has  not  infre- 
quently made  a  way  for  itself  by  fusing  or  dissolving  and 


186  STRUCTURAL  AND  FIELD  GEOLOGY 

absorbing  certain  rocks,  such  as  coal  and  limestone,  and  even 
much  less  readily  reduced  materials. 

It  is  obvious,  therefore,  that  molten  masses  which  have 
cooled  and  consolidated  within  the  earth's  crust  must  vary 
in  shape  according  to  the  form  of  the  passages  and  cavities 
which  have  been  opened  for  them.  Consequently,  from  the 
point  of  view  of  tectonic  geology,  intrusive  eruptive  rocks  are 
grouped  under  certain  more  or  less  definite  structural  types. 
It  must  not  be  supposed,  however,  that  each  individual 
intrusive  mass  necessarily  belongs  wholly  to  one  or  other  of 
these  typical  structures.  Not  infrequently,  as  we  shall  learn, 
several  different  types  may  be  represented  by  one  and  the 
same  eruptive  rock-mass.  The  more  important  structures 
recognised  by  geologists  are  as  follows  : — Batholiths ;  Lacco- 
liths and  Sills  or  Sheets ;  Necks  or  Plugs ;  and  Dykes  and 
Veins. 

i.  BATHOLITHS 

The  term  Batholith  is  applied  to  an  intrusive  mass,  of 
deep-seated  origin,  which  seems  to  occupy  an  amorphous  or 
irregular  shaped  cavity,  usually  of  large  dimensions,  often, 
indeed,  measuring  several  miles  in  diameter  (Fig.  63).  Batho- 


FIG.  63. — DIAGRAMMATIC  SECTION  ACROSS  A  BATHOLITH. 

liths  consist  usually  of  some  granitoid  holocrystalline  rock, 
as  granite,  syenite,  diorite,  gabbro,  dolerite,  etc.  Occasionally, 
however,  quartz-porphyry  occurs  in  similar  masses.  But  the 
most  characteristic  batholiths  are  unquestionably  the  deep- 
seated  plutonic  masses  of  granitoid  holocrystalline  rocks,  of 
which  granite  itself  may  be  taken  as  the  type. 

The  petrographical  character  of  granite,  not  less  than  the 
phenomena  presented  by  the  rocks  it  traverses,  are  sufficient 
proof  of  its  deep-seated  origin.  Granitic  intrusions  range  in 
age  from  the  oldest  period  recognised  by  geologists  down  to 


i.  JUNCTION  OF  GRANITE  WITH  FINE-GRAINED  GNEISS.    Natural  size. 


2.  VEIN  OF  GRANITE  TRAVERSING  GNEISS.     Natural  size. 

[To  face  page  186. 


STRUCTURE  OF  ERUPTIVE  ROCKS  187 

Tertiary  times.  By  far  the  larger  number,  however,  date 
back  to  Palaeozoic  and  Archaean  ages— very  few,  indeed, 
being  referable  to  Mesozoic  and  Cainozoic  horizons.  From 
this  we  are  not  justified  in  concluding  that  intrusions  of  granite 
were  of  more  common  occurrence  in  the  earlier  than  in  the 
later  stages  of  the  world's  history.  Granite,  being  of  plutonic 
origin,  can  only  appear  at  the  surface  as  the  result  of  long- 
continued  and  profound  denudation,  and  its  relative  age  is 
fixed  by  that  of  the  rocks  it  traverses.  If  the  surrounding 
rocks  be  of  Palaeozoic  age,  all  we  can  say  is  that  the  intrusion 
is  of  later  date  than  these — but  how  much  later  we  cannot 
tell.  For,  obviously,  much  denudation  must  have  taken 
place  before  the  granite  could  become  exposed  at  the 
surface.  It  may  originally  have  risen  to  a  much  higher 
geological  horizon — all  evidence  of  this  having  been 
destroyed  by  the  complete  removal  of  the  overlying 
younger  rocks,  and  those  portions  of  the  batholith  itself  by 
which  these  may  have  been  penetrated.  As  every  granite 
intrusion  must  in  this  way  have  traversed  older  rocks  before  it 
could  reach  superincumbent  younger  rocks,  we  might  have 
expected  to  find  batholiths  most  frequently  associated  with 
the  former,  although  many  may  really  belong  to  a  much 
later  date. 

Collateral  evidence  sometimes  enables  the  geologist  to  fix 
the  approximate  age  of  a  batholith.  When,  for  example,  the 
rock  is  seen  traversing  Carboniferous  strata,  while  fragments 
of  it  are  enclosed  in  beds  of  early  Permian  age,  we  may  infer 
that  th£  intrusion  probably  took  place  towards  the  close  of 
Carboniferous  times. 

Along  its  junction  with  adjacent  rocks,  granite  is  often 
finer  grained  than  elsewhere  in  the  same  mass,  as  if  the 
molten  magma  had  become  chilled  by  contact  with  its 
surrounding  walls,  and  cooled  too  rapidly  to  allow  the 
constituent  minerals  to  attain  fuller  development  (see  Plate 
XLIII.  i).  Not  infrequently,  however,  the  rock  is  as  coarsely 
crystalline  along  its  margin  as  towards  the  centre  of  the 
mass.  In  such  cases  we  may  suppose  that  the  surrounding 
rocks  were  so  highly  heated  as  to  have  no  chilling  influence. 
Although  the  junction  between  granite  and  the  rocks  invaded 
by  it  is  usually  so  clearly  defined  that  a  knife-edge  may  be 


188  STRUCTURAL  AND  FIELD  GEOLOGY 

laid  upon  it,  yet  this  is  not  always  the  case.  Occasionally, 
the  eruptive  rock  seems  to  merge  insensibly  into  the  other, 
and  no  line  of  demarcation  is  visible.  Again,  it  sometimes 
happens  that  when  granite  has  invaded  schists  or  slates  it  has 
penetrated  these  by  a  kind  of  leaf-by-leaf  injection  —  the 
liquid  rock  having  insinuated  itself  in  excessively  thin  sheets 
and  veins  along  planes  of  foliation  or  cleavage.  Under  such 
conditions  the  invaded  rocks  are  so  intimately  mixed  with 
granite  and  so  highly  metamorphosed,  that  it  is  often  very 
difficult  to  distinguish  between  them  and  the  invading  rock. 
The  alternating  leaves  of  granite  and  schist  combine,  in 
short,  to  produce  a  rock  which  has  the  aspect  of  a  gneiss 
into  which  the  granite-mass  seems,  as  it  were,  to  graduate. 

Now  and  again  the  marginal  area  of  a  granitic  batholith 
contains  more  or  less  numerous  angular  and  subangular 
fragments,  slabs,  reefs,  and  blocks  of  schistose  or  other  rocks. 
Such  inclusions,  or  xenoliths,  as  they  are  called,  are  not  to  be 
confounded  with  the  relatively  fine-grained,  dark  basic  secre- 
tions described  in  Chapter  III.,  as  characteristic  of  many 
granites.  On  the  contrary,  they  have  obviously  been  torn 
from  the  rocks  abutting  upon  the  granite  and  enclosed  in  it 
at  the  time  of  its  intrusion.  It  may  be  added  that  granitic 
batholiths  not  infrequently  show  a  kind  of  foliated  or  flow 
structure  near  their  margins — the  constituent  minerals  being 
arranged  roughly  parallel  to  the  junction-line.  This  may 
indicate  an  actual  fluidal  movement,  or  it  may  simply  be  the 
result  of  hydrostatic  pressure,  exerted  by  the  mass  of  the 
granite  itself. 

Batholiths  are  often  rudely  circular  or  elliptical  in  groundplan,  and 
seem,  in  some  cases,  to  rise  up  vertically,  as  if  they  occupied  an  enormous 
pipe  or  funnel.  The  rocks  surrounding  such  batholiths  have  no  appear- 
ance of  having  been  thrust  aside  to  make  room  for  the  intrusive  mass. 
To  explain  this,  it  has  been  suggested  that  the  rocks  which  formerly 
occupied  the  site  of  the  batholith  may  have  been  melted  up  and 
assimilated  by  the  granite.  That  absorption  to  some  extent  has  actually 
occurred,  in  some  cases  at  least,  is  suggested  by  the  fact,  already 
mentioned,  that  granite  occasionally  merges  gradually  into  the  rocks 
against  which  it  abuts.  It  is  further  noteworthy,  in  this  connection,  that 
a  difference  of  chemical  composition  has  now  and  again  been  detected 
between  the  granite  near  its  margin  and  towards  the  centre  of  the  mass. 
It  would  seem,  however,  that  xenoliths  are  confined  as  a  rule  to  the  marginal 
areas  of  a  batholith,  whereas,  had  the  rocks  which  formerly  occupied  its 


STRUCTURE  OF  ERUPTIVE  HOCKS  189 

site  been  broken  up  and  absorbed,  one  might  have  expected  to  meet 
with  occasional  detached  xenoliths  throughout  the  whole  mass,  while  the 
granite  itself  ought  to  have  varied  more  or  less  markedly  in  composition, 
considering  the  very  different  kinds  of  rock-material  which  it  must  have 
absorbed.  Granitic  batholiths  do,  indeed,  sometimes  vary  remarkably  as 
regards  their  petrographical  character :  but  such  variations  are  cases  of 
magmatic  differentiation,  and  appear  to  be  due  to  the  way  in  which  the 
mineral  constituents  separated  out  from  the  original  magma — so  that  in 
some  places,  particularly  towards  the  margin  of  a  mass,  the  rock  is  often 
more  basic  than  towards  the  centre.  It  cannot  be  said,  therefore,  that 
the  hypothesis  of  absorption  is  in  all  cases  a  satisfactory  explanation  of 
the  phenomena.  Quite  recently,  however,  Dr  Sederholm  has  shown  that 
in  Finland  a  process  of  fusion  and  assimilation  has  actually  taken  place. 
Certain  large  areas  of  schistose  rocks — most  of  them  of  acid  composition, 
but  some  markedly  basic — have  been  melted  and  transformed  into  granite 
and  granite-gneiss,  scattered  through  which  more  or  less  numerous 
fragments  of  the  basic  rocks  are  conspicuous.  (Seefostea,  p.  222.)  Other 
geologists  have  speculated  on  the  possibility  of  rock-masses  having  been 
pushed  up  or  even  blown  out  in  fragments  by  vapours  escaping  from 
a  batholith.  But  this  would  imply  that  all  batholiths  must  have 
had  communication  with  the  exterior,  which  can  hardly  be  admitted. 
On  the  contrary,  there  is  no  reason  to  doubt  that  many  extensive  masses 
of  granite  never  had  any  communication  with  the  surface,  but  cooled  and 
consolidated  at  abyssal  depths.  On  the  other  hand,  the  peculiar  manner 
in  which  granite  and  other  granitoid  rocks  are  sometimes  associated 
with  effusive  rocks,  leads  to  the  well-grounded  belief  that  such  batholiths 
are  the  cores  or  roots  of  ancient  volcanoes.  As  an  example  may  be  cited 
the  augite  granite  of  the  Cheviot  Hills,  which  is  closely  associated  with  a 
great  series  of  lavaform  rocks  and  tuffs.  In  such  cases  it  must  be 
admitted  that  some  batholiths  are  of  less  deep-seated  origin  than  others. 

It  must  not  be  supposed  that  granite  always  occurs  in 
boss-like  masses.  On  the  contrary,  it  frequently  appears  in 
the  form  of  extensive  sheets  of  very  variable  thickness — and 
it  may  well  be  doubted  whether  many  of  the  plutonic  masses 
which  have  been  supposed  to  occupy  more  or  less  vertical 
funnel-like  cavities  are  really  of  this  character.  So  far  as  one 
can  tell  from  what  is  exposed  at  the  surface,  the  so-called 
"  bosses "  may  simply  be  partially  exposed  sheets,  some  of 
which,  however,  must  have  a  thickness  of  several  thousand 
feet.  A  sheet-like  structure  is  suggested  by  the  fact  that,  far 
removed  from  the  margin  of  a  granite  area,  inliers  of  the 
same  rock  are  not  infrequently  revealed  in  the  beds  of  streams 
which  have  cut  their  way  down  through  a  great  thickness 
of  the  metamorphosed .  rocks  surrounding  the  central  mass. 


190 


STRUCTURAL  AND  FIELD  GEOLOGY 


In  such  cases  the  granite  may  either  be  of  the  nature  of  a 
laccolith,  or  thick  sill  or  sheet,  following  an  irregular  course 
through  the  rocks  among  which  it  has  been  intruded  (Fig.  64), 


FIG.  64. 

or  the  central  mass  may  be  a  true  boss  from  which  sheets 
extend  outwards  at  different  levels  and  in  different  directions 
(Fig.  65). 

The  rocks  for  some  distance  around    a  mass  of  granite 


FIG.  65. 

are  usually  more  or  less  highly  metamorphosed,  and  traversed 
by  numerous  dykes  and  veins  (Plate  XLIII.  2)  proceeding 
from  the  eruptive  rock,  as  will  be  more  fully  explained  in  the 
sequel. 

Rocks  of  a  more  basic  character  than  granite,  such  as 
diorite,  syenite,  gabbro,  dolerite,  etc.,  not  infrequently  occur  in 
the  form  of  great  batholiths — usually  on  a  smaller  scale, 
however,  than  the  more  typical  granitic  masses.  Like  the 
latter,  they  seem  sometimes  to  occupy  the  place  of  rocks 
which  may  either  have  been  absorbed  or  pushed  up  and 
blown  out.  In  other  cases  they  are  of  the  nature  of  gigantic 
laccoliths.  Mr  Harker  has  described  some  in  the  island  of 
Skye  which  attain  a  thickness  of  3000  feet.  Not  a  few  basic 
batholiths  are  apparently  of  less  deep-seated  origin  than 
granite,  and  although  many  may  never  have  communicated 
with  the  surface,  yet  there  are  good  grounds  for  believing 
that  some  of  them,  at  least,  are  the  roots  or  cores  of  old 
volcanoes,  the  effusive  products  of  which  are  grouped 
immediately  around  them.  As  examples  of  the  kind,  we 
may  cite  the  bosses  of  diabase,  etc.,  which  appear  in  the 


STRUCTURE  OF  ERUPTIVE  ROCKS  191 

midst  of  the  lavas  and  tuffs  of  the  Sidlaw  Hills,  the  Ochil 
Hills,  and  other  similar  ranges  in  Central  Scotland.  From 
batholiths  of  all  kinds  proceed  more  or  less  numerous 
apophyses — sheets  or  sills,  dykes,  and  veins — which  penetrate 
the  contiguous  rocks  often  for  considerable  distances. 

2.  LACCOLITHS  AND  SILLS 

The  name  Laccolith  has  been  given  to  certain  remark- 
able masses  of  intrusive  rock,  which  have  been  described  by 
Mr  G.  K.  Gilbert  as  occurring  in  the  Henry  Mountains  of 
Southern  Utah.  The  same  type  of  structure  has  since  been 
recognised  in  the  Elk  Mountains,  and  elsewhere  in  North 
America.  As  these  laccoliths  are  of  late  Tertiary  age,  many 
of  them  are  still  in  an  excellent  state  of  preservation,  and  the 
phenomena  they  present  enable  us  to  understand  more 
readily  the  conditions  under  which  certain  of  our  own 
intrusive  rock-masses  may  have  come  into  existence.  The 
general  structure  of  a  laccolith  is  illustrated  in  the  accompany- 
ing diagram  (see  Fig.  66).  It  will  be  observed  that  the 


FIG.  66. — LACCOLITH. 

intrusive  rock  is  lenticular  in  shape,  that  it  sends  out  sheets, 
dykes,  and  veins  into  the  contiguous  strata,  and  is  in  connec- 
tion with  a  subjacent  pipe-like  feeder.  According  to  Mr 
Gilbert,  the  molten  rock  has  risen  through  this  vertical  pipe 
or  fissure,  but,  being  unable  to  burst  across  the  superincumbent 
beds,  has  insinuated  itself  between  the  strata,  lifted  these  up, 


192  STRUCTURAL  AND  FIELD  GEOLOGY 

and  thus  produced  a  dome-like  elevation  at  the  surface. 
Proceeding  from  such  a  laccolith  are  more  or  less  numerous 
intrusions — some  of  which  have  been  injected  along  the 
bedding-planes,  while  others  cut  across  the  fissured  strata  at 
all  angles.  While  laccoliths  sometimes  occur  singly,  they  more 
usually  appear  in  clusters — the  presence  of  each  cluster  being 
indicated  by  a  dome-shaped  mountain.  The  number  of 
individual  laccoliths  in  a  cluster  is  variable — sometimes  there 
are  no  more  than  two,  in  other  cases  there  may  be  a  score, 
the  largest  number  recognised  in  one  group  being  thirty. 

Let  us  now  see  what  light  this  American  type  of  intrusive 
rock  throws  upon  the  phenomena  of  the  sills  or  intrusive 
sheets  which  are  of  such  common  occurrence  in  our  own  country. 
Sills  are  eruptive  masses  which  have  usually  been  intruded 
along  planes  of  stratification,  and  hence  they  tend  to  assume 
a  more  or  less  regularly  bedded  aspect.  The  plane  along 
which  intrusion  has  taken  place  is  not  necessarily,  however,  a 
plane  of  bedding.  Some  sills  have  followed  planes  of  slaty 
cleavage  and  foliation,  while  others  continue  for  longer  or 
shorter  distances  along  lines  of  fracture.  But  certainly  the 
most  typical  examples  are  met  with  amongst  stratified  rocks, 
with  which  they  have  the  appearance  of  being  interbedded. 
Almost  any  kind  of  eruptive  rock  may  assume  the  form  of 
a  sill,  although  the  deeper-seated  granitoid  rocks,  such  as 
granite,  syenite,  diorite,  etc.,  appear  less  frequently  in  sheet- 
like  masses  than  the  hypabyssal  dolerites,  basalts,  andesites, 
etc.  Perhaps  the  most  typical  examples  of  the  true  sill  are 
those  which  occur  so  frequently  among  the  Palaeozoic  strata  of 
these  islands — the  sills  of  the  Carboniferous  areas  being 
particularly  well  known.  It  may  suffice,  therefore,  to  give  a 
short  account  of  the  latter. 

We  may  note,  then,  that  a  sill,  although  it  may  seem  to 
be  interbedded  as  a  member  of  one  consecutive  series  of 
strata,  does  not  exactly  conform  to  the  immediately  overlying 
and  underlying  beds  (Fig.  67).  Followed  along  the  outcrop, 
it  is  found  now  and  again  to  leave  the  plane  upon  which  it 
first  appeared — either  rising  to  a  slightly  higher  or  descending 
to  a  slightly  lower  level.  Or  it  may  suddenly  break  across 
a  considerable  thickness  of  strata  and  proceed  thereafter  along 
a  totally  different  horizon.  Not  infrequently  it  contains 


[To  face  page  198. 


STRUCTURE  OF  ERUPTIVE  ROCKS  193 

fragments  torn  from  the  contiguous  rocks ;  occasionally, 
indeed,  large  slabs  or  sheets  of  the  invaded  strata  have  been 
caught  up  and  enclosed  in  the  eruptive  rock,  and  such 
fragments  are  invariably  much  baked  and  altered.  Many 
thick  sills  divide  into  two  or  several  subordinate  sheets, 
each  more  or  less  closely  following  a  plane  of  bedding.  Often, 
also,  dykes  and  veins  proceed  from  sills  into  the  adjacent 
rocks.  This  is  frequently  the  case  when  a  thick  sill  divides, 
the  separate  sheets  being  often  connected  by  one  or  more 
dykes  passing  across  the  intervening  strata.  But  the  whole 
complex  of  sheets  and  dykes  has  obviously  been  intruded 
at  one  and  the  same  time.  Each  independent  sill  or  group 


FIG.  67.— SILL  OR  INTRUSIVE  SHEET. 

6,  dolerite  ;  s,  sandstones  and  shales. 

of  subordinate  and  associated  sheets  is  doubtless  connected 
with  one  or  more  vertical  pipes  or  feeders,  although  these 
have  not  often  been  seen  in  section. 

The  sills  of  our  Carboniferous  areas  consist  principally 
of  basic  rocks,  mainly  dolerites  and  basalts.  Some  of  these 
are  not  more  than  a  few  feet  or  yards  in  thickness  ;  others 
may  reach  and  even  exceed  1 50  feet.  They  are  all  lenticular 
in  shape,  some  dying  out  more  rapidly  than  others.  At  and 
near  its  junction  with  the  overlying  and  underlying  strata, 
a  sill  is  almost  invariably  finer  grained  than  towards  the 
centre  of  the  mass.  Along  the  actual  line  of  contact  it  is 
frequently  compact  and  even  markedly  vitreous.  In .  the 
case  of  thin  sheets  the  texture  is  usually  finer  grained,  and 
the  rock  may  contain  much  glassy  base  throughout.  The 
thicker  sills,  on  the  other  hand,  tend  to  be  coarser  grained 
and  holocrystalline.  Vapour  cells  are  usually  absent,  although 
now  and  again  sporadic  areas  of  vesicles  appear;  but  these 
are  never  so  plentiful  as  to  impart  a  scoriaceous  aspect  to 
the  rock. 

N 


194  STRUCTURAL  AND  FIELD  GEOLOGY 

The  strata  in  contact  with  a  sill  never  fail  to  afford 
evidence  of  having  been  subjected  to  the  action  of  heat. 
Both  overlying  and  underlying  strata  are  invariably  affected, 
the  alteration  at  the  point  of  contact  being  often  excessive. 
But  the  alteration  never  extends  so  far  from  the  eruptive 
rock  as  in  the  case  of  granitic  intrusions.  Some  account  of 
these  and  other  changes  produced  by  sills  will  be  considered 
in  the  sequel. 

Evidence  is  not  wanting  to  show  that  sills  have  new  and  again  melted 
up  and  absorbed  some  of  the  rocks  with  which  they  have  come  in 
contact.  In  the  Scottish  coal-fields,  for  example,  they  have  not  infre- 
quently eaten  up  thick  seams  of  coal  and  black  shale  which  they  have 
followed  as  lines  of  least  resistance.  In  cases  of  this  kind  the  basalt- 
rock  is  usually  much  altered,  becoming  bleached  white  or  yellow,  and 
assuming  a  dull,  clay-like  aspect  ("white  trap")-  Limestones  are  occa- 
sionally demolished  in  the  same  way,  and  their  place  taken  by  sills. 
But  to  what  extent  other  kinds  of  rock  may  have  been  absorbed  is  quite 
uncertain.  The  rock  of  a  thick  sill  not  infrequently  varies  in  petro- 
graphical  character,  being  in  some  places  less  basic  than  the  normal. 
But  while  such  variations  may  be  the  result  of  absorption  of  extraneous 
materials,  they  seem  just  as  likely  to  be  due  to  magmatic  differentiation, 
the  more  basic  areas  having  separated  out  during  the  earlier  stages  of 
cooling.  It  may  be  mentioned,  however,  that  not  infrequently  the  intrusion 
of  a  sheet  into  a  series  of  strata  lying  between  two  seams  of  coal  or 
ironstone  has  not  apparently  increased  the  distance  between  those  seams, 
as  it  might  have  been  expected  to  do.  In  the  neighbourhood  of 
Dalmellington,  Ayrshire,  for  example,  thick  sheets  of  basalt  have  been 
here  and  there  intruded  amongst  a  series  of  sandstones  and  shales 
which  come  between  two  conspicuous  seams — a  coal  and  a  blackband 
ironstone —the  distance  from  the  one  seam  to  the  other  being  quite 
well  known.  In  some  places  only  one  sheet  is  present ;  in  other  parts 
of  the  same  neighbourhood  there  are  two,  while  at  intermediate  points 
the  pits  may  encounter  none  at  all.  Yet,  in  sinking  shafts,  the  miners 
always  reach  the  seam  (ironstone)  they  are  in  search  of  at  the  estimated 
depth  below  the  coal,  no  matter  whether  sills  are  present  or  not.  In 
short,  the  distance  between  the  two  given  horizons  is  neither  increased 
nor  diminished  by  the  presence  or  absence  of  the  intrusive  sheets.  It 
seems  difficult  to  account  for  such  phenomena  (and  many  similar  instances 
occur),  except  on  the  supposition  that  molten  rock  has  the  power  of 
absorbing  rock-material,  and  that,  as  Mr  Clough  has  suggested,  there 
may  have  been  a  general  circulation  in  the  mass  which  reduced  all  parts 
of  the  mixture  to  a  uniform  composition.  But  much  petrographical  and 
chemical  research  must  be  done  before  a  question  of  this  kind  can  be 
settled  satisfactorily. 

Sills  often  appear  in  large  numbers  in  regions  of  former  volcanic 
activity.  Those  associated  with  the  Carboniferous  strata  of  Scotland  are 


STRUCTURE  OF  ERUPTIVE  ROCKS  195 

a  case  in  point— for  volcanic  action  was  manifested  in  that  country  again 
and  again  during  Carboniferous  times.  It  is  probable,  therefore,  that 
most  of  the  sills  referred  to  were  contemporaneous  in  origin  with  the 
lavas  and  tuffs  of  that  period.  Some  of  them  may  have  been  intruded 
before  the  eruptive  forces  had  succeeded  in  establishing  any  communica- 
tion with  the  surface  ;  others  may  well  be  synchronous  with  the  full 
development  of  volcanic  activity  ;  while  yet  others  may  mark  the  dying 
out  of  that  action,  when  the  eruptive  energy  was  insufficient  to  pump 
lava  to  the  surface.  The  out-cropping  of  these  sills  is,  of  course,  the 
result  of  the  general  folding  and  denudation  of  the  strata.  But  no  one 
who  compares  the  phenomena  they  present  with  those  exhibited  by  the 
well-preserved  laccoliths  of  North  America,  can  doubt  that  the  older  and 
younger  structures  have  much  in  common,  and  may  well  have  had  the 
same  origin.  Sills  which  crop  out  at  the  surface  so  as  to  form  lofty 
mural  escarpments  have  been  proved  in  many  cases  to  wedge  out  down- 
wards, and  now  and  again  their  "feeders"  have  been  recognised.  In 
such  cases  it  is  not  hard  to  reconstruct  the  original  condition  of  the 
intrusion  (Fig.  68).  Indeed,  it  may  be  said  that  most  of  the  salient 


FIG.  68.— DIAGRAM  OF  A  SILL,  SHOWING  ITS  FORMER  EXTENSION  AS  A 

LACCOLITH. 

features  of  the  American  laccoliths  are  reproduced  by  the  sills  of  our  own 
country.  The  latter  occur  singly  or  in  groups  just  as  the  former  do.  A 
laccolith  may  divide,  as  it  were,  into  two  or  more  wedge-shaped  and 
approximately  parallel  sheets,  and  many  Scottish  sills  behave  in  the  same 
way.  So  again  from  laccoliths  and  sills  alike  veins  and  dykes  are  pro- 
truded into  the  contiguous  strata.  There  is  no  evidence,  however,  that 
would  lead  us  to  infer  that  the  Scottish  sills  affected  the  configuration  of 
the  surface,  forming  dome-shaped  elevations  in  the  same  way  as  the 
laccoliths  of  the  Henry  Mountains. 

From  the  foregoing  account  of  batholiths  and  laccoliths  it 
is  obvious  that  no  hard-and-fast  line  can  be  drawn  between 
the  two :  for  many  batholiths  assume  the  laccolitic  habit. 
Batholiths,  however,  usually  occur  on  a  much  larger  scale 
than  laccoliths,  and  are,  upon  the  whole,  of  more  deep- 
seated  origin,  while  now  and  again  they  seem  to  occur 
as  bosses  occupying  enormous  vertical  pipes  or  funnels. 


196  STRUCTURAL  AND  FIELD  GEOLOGY 

3.  NECKS 

Necks  are  pipes  or  conduits  of  eruption — the  throats,  in 
short,  of  old  volcanoes.  They  are  filled  either  with  crystalline 
rock  or  fragmental  materials,  or  with  both.  They  are  of  less 
deep-seated  origin  than  batholiths  ;  indeed,  portions  of  the  old 
volcanic  cone  are  still  to  be  seen  surrounding  a  neck  in  some 
cases.  As  a  rule,  however,  the  cones  have  been  entirely 
demolished — only  the  plugged-up  vents  remaining.  Not  a  few 
of  these  seem  to  represent  very  small  volcanoes — the  products 
of  single  eruptions,  like  that  which,  in  1538,  gave  birth  to  the 
tuff  and  cinder  cone  of  Monte  Nuovo  (Bay  of  Baiae).  Others, 
again,  are  obviously  the  relics  of  much  more  important 
volcanoes,  from  which  were  discharged  not  only  fragmental 
materials  but  streams  of  lava.  Between  necks  of  this  kind 
and  certain  bosses  no  hard-and-fast  line  can  be  drawn.  Some 
of  the  latter,  as  we  have  seen,  appear  to  have  had  communica- 
tion with  the  surface,  and  these,  therefore,  might  equally  well  be 
described  as  necks.  That  term,  however,  ought  rather  to  be 
reserved  for  the  less  important  pipes  or  funnels  of  eruption — 
most  of  which,  indeed,  represent  only  the  uppermost  or 
terminal  portions  of  such  pipes.  For,  even  in  the  case  of  the 
most  highly  denuded  neck,  we  have  no  reason  to  suppose  that 
the  portion  remaining  occurred  at  any  great  depth  below  the 
base  of  the  old  volcanic  cone  to  which  it  led.  It  is  conceivable 
that,  could  we  trace  an  important  neck  downwards,  we  should 
find  it  gradually  assume  the  character  of  a  more  or  less 
funnel-shaped  boss,  and  this  in  its  turn  might,  at  a  greater 
depth  still,  expand  into  a  yet  more  extensive  batholith.  It 
would  seem,  therefore,  as  if  the  structure  now  presented  by 
many  an  old  focus  of  eruption,  may  have  been  determined  by 
the  degree  of  denudation  which  it  has  experienced.  With  a 
minimum  amount  of  erosion  we  have  the  cone  of  the  extinct 
volcano,  still  recognisable  as  such.  Increased  erosion  removes 
the  cone,  and  then  only  a  neck  remains;  until  after  some 
prolonged  period  the  whole  region  becomes  so  reduced  that 
the  batholith  or  more  deeply-seated  portion  is  laid  bare. 

Seen  in  groundplan,  typical  necks  tend  to  be  more  or  less 
circular  or  elliptical  in  form,  but  they  are  frequently  irregular. 
Occasionally,  however,  such  irregular  shapes  are  suggestive 


STRUCTURE  OF  ERUPTIVE  ROCKS 


197 


of  two  or  more  closely  adjacent  necks  having  coalesced.  Not 
infrequently,  fissures,  filled  with  agglomerate  or  tuff,  pass 
outwards  from  a  neck  into  the  adjacent  rocks.  More  remark- 
able than  these,  however,  are  certain  vertical  fissures  of 
eruption  which  occur  independently,  or  seem,  at  least,  to  have 
no  connection  with  necks  or  pipes.  At  the  surface,  these 
appear  in  groundplan  as  long,  lenticular  ribbons  or  belts,  or 
they  may  expand  and  contract  irregularly.  They  are  filled 
with  fragmental  materials,  and  thus  might  be  tersely  described 
as  agglomerate-dykes.  Fissures  of  eruption  of  this  kind  are 
not  common,  and  seem  to  be  confined  to  regions  where 
volcanic  rocks  are  well  developed.  Isolated  examples  occur 
in  the  Sidlaw  Hills  and  in  South  Ayrshire,  and  they  are 
met  with  likewise  in  the  Cheviot  Hills.  Necks  often  appear 
upon  a  line  of  fault  or  dislocation,  but  in  many  cases  no  such 
connection  can  be  traced.  Although  they  now  and  again 
occur  singly,  they  more  usually  cluster  in  groups  within  a 
limited  area.  They  vary  much  in  size — some  measuring  only 
a  few  yards  across,  while  others  may  be  several  hundred  yards 
in  diameter ;  exceptionally,  they  may  reach  or  even  exceed  a 
mile  in  width.  They  usually  form  more  or  less  abrupt  knolls 
or  isolated  hills,  which  vary  in  shape  according  to  the  nature 
of  the  materials  of  which  they  are  composed.  Many  are 
more  or  less  conical ;  others  are  somewhat  steep  and  not 
infrequently  craggy  ;  while  yet  others  are  smooth  and  rounded. 
The  rock  occupying  a  neck  may  be  crystalline,  as  basalt, 
andesite,  phonolite,  quartz-porphyry,  felsite,  etc.  (see  Fig.  69)  ; 


FIG.  69.— NECK  OCCUPIED  BY 
CRYSTALLINE  IGNEOUS  ROCK. 


FIG.  70. — NECK  OCCUPIED  BY 
AGGLOMERATE. 


or  it  may  consist  of  fragmental  materials,  as  agglomerate  or 
tuff  (see  Fig.  70),  or  both  fragmental  and  massive  crystalline 
igneous  rocks  may  be  present  (see  Fig.  71).  Frequently  the 


198  STRUCTURAL  AND  FIELD  GEOLOGY 

fragmental  materials  are  extremely  coarse — an  aggregate  of 
angular  and  subangular  blocks  and  smaller  stones  in  a  matrix 
of  finely  comminuted  debris,  which  may  be  meagre  or  relatively 

abundant.  All  the  frag- 
ments may  consist  of 
crystalline  igneous  rock 

°^  one  or  more  kinds,  or 
these  may  be  commingled 
with   the   debris   of  sedi- 
FIG.    71.— NECK    OCCUPIED    BY    AGGLO-      mentary  rocks — the   rela- 

MERATE     AND      CRYSTALLINE      IGNEOUS        tive  proportion  of  ignCOUS 

ROCK.  i         j. 

and  sedimentary  mate- 
rials varying  indefinitely.  Sometimes  the  contents  of  a  neck 
consist  of  derivative  rocks  only,  as  sandstone,  shale,  limestone, 
ironstone,  coal,  etc.  In  necks  composed  mostly  or  exclusively 
of  igneous  materials,  large  broken  crystals  of  various  volcanic 
minerals  sometimes  occur,  as  hornblende,  augite,  biotite,  sani- 
dine,  pyrope,  etc.  Still  more  remarkable  is  the  appearance 
in  some  tuff-necks  of  abundant  small  and  larger  fragments 
of  coniferous  wood.  Although  the  fragmental  materials 
are  usually  somewhat  coarse,  yet  not  infrequently  these  are 
associated  in  the  same  neck  with  areas  of  much  finer  grained 
tuff;  while  in  some  cases  the  whole  neck  consists  of  fine  tuff, 
which  now  and  again  has  been  so  altered  as  to  assume  a 
crystalline  or  subcrystalline  aspect. 

The  agglomerate  and  tuff  often  exhibit  more  or  less 
distinct  traces  of  a  ceritroclinal  dip — the  materials  being  rudely 
bedded  around  the  marginal  area  and  inclined  inwards  towards 
the  centre,  where  all  trace  of  bedding  is  usually  lost,  although 
occasionally  the  coarse  material  appears  roughly  arranged  in 
nearly  vertical  lines.  Not  infrequently  it  is  about  the  centre 
of  a  neck  that  the  larger  blocks  and  stones  are  most  abundant ; 
but  in  many  necks  no  such  aggregation  can  be  traced. 

Massive  dykes  and  branching  veins  of  basalt  or  other 
crystalline  igneous  rock  often  pierce  and  ramify  through  the 
agglomerate  and  tuff.  These  may  be  confined  to  the  neck 
itself,  or  pass  outwards  into  the  contiguous  strata.  The 
massive  rock  which  often  completely  fills  a  neck  is  usually 
traversed  by  well-marked  horizontal  jointing,  but  in  the  case 
of  large  necks  these  joints  are  often  confined  to  the  marginal 


STRUCTURE  OF  ERUPTIVE  ROCKS  199 

area,   the   rock   towards   the   centre   being,   as   a   rule,   very 
irregularly  jointed. 

The  strata  in  immediate  contact  with  a  neck  are  often 
bent  over  suddenly,  so  as  to  dip  abruptly  against  the  old 
pipe  of  eruption — not  infrequently,  indeed,  they  are  quite 
vertical,  and  sometimes  much  jumbled  and  broken,  large 
blocks,  slabs,  and  reefs  having  been  detached  or  partially 
detached  from  the  walls  of  the  neck,  so  as  to  become 
enclosed  wholly  or  partially  in  the  tuff  and  agglomerate, 
while  irregular  veins  of  tuff  pass  outwards  into  the  contiguous 
strata  as  if  filling  rents  and  fissures.  Now  and  again,  so 
great  is  the  confusion  that  it  is  hard  to  follow  the  actual 
junction  between  the  neck  and  the  contiguous  rocks.  In 
such  cases  it  seems  as  if  the  wall  of  the  old  funnel  had 
collapsed  and  fallen  in.  The  effect  of  heat  upon  the  rocks 
abutting  upon  a  neck  are  sometimes  very  notable — sand- 
stones for  a  few  yards  away  being  converted  into  quartzite, 
and  shales  baked  into  a  kind  of  porcellanite.  On  the  other 
hand,  not  infrequently  no  alteration  of  any  kind  can  be  seen, 
coal  having  sometimes  been  mined  close  up  against  a  neck 
without  showing  any  trace  of  having  been  subjected  to  the 
action  of  heat  In  other  cases,  coal  has  been  rendered  quite 
useless  for  many  yards  away  from  a  neck,  changed  in  fact 
into  a  soft,  sooty  substance.  The  amount  of  alteration  pro- 
duced bears  no  relation  to  the  size  of  a  neck ;  for  while 
much  change  may  occur  round  a  small  one,  little  or  no 
alteration  may  be  visible  round  one  of  much  larger 
dimensions. 

Explanation  of  Phenomena. — The  necks  described  above  obviously 
indicate  the  sites  of  former  volcanoes.  Many  occur  along  lines  of 
dislocation,  just  as  is  apparently  the  case  with  not  a  few  volcanoes  at 
the  present  time.  On  the  other  hand,  a  large  number  of  necks  seem  to 
have  no  connection  with  any  lines  of  weakness,  and  such  pipes  of 
eruption,  therefore,  must  have  been  blown  or  blasted  out  by  escaping 
vapours.  Many  necks  probably  indicate  small  puys — products  of  a  single 
eruption,  from  which  only  loose  ejecta  were  emitted.  From  others,  one 
or  more  flows  of  lava  have  taken  place.  When  the  tuff  and  agglomerate 
of  a  neck  consist  wholly  or  largely  of  igneous  materials,  it  is  obvious 
that  molten  matter  must  have  risen  in  the  throat  of  the  old  puy,  although 
it  may  never  have  flowed  out  as  a  lava.  It  is  quite  possible,  however, 
or  even  probable,  that  lava-streams  may  have  proceeded  from  many 
necks,  around  which  no  remains  of  such  flows  now  exist.  Subsequent 


200  STRUCTURAL  AND  FIELD  GEOLOGY 

denudation  would  well  account  for  their  disappearance,  for  not  only 
have  the  volcanic  cones  been  removed,  but  the  surface  upon  which  these 
were  built  up  has  also  often  been  carried  away.  In  the  case  of  those 
necks  which  contain  no  igneous  materials,  but  are  filled  exclusively  with 
the  debris  of  derivative  rocks,  it  is  clear  that  if  at  the  time  of  eruption 
molten  matter  was  present  at  all,  it  could  only  have  been  at  a  relatively 
great  depth.  The  character  of  the  debris,  at  all  events,  shows  that  only 
explosive  vapours  escaped  by  such  pipes  and  funnels.  While  there  is 
reason  to  believe  that  some  necks  may  represent  subaerial  volcanoes, 
not  a  few  are  certainly  of  subaqueous  origin.  In  the  former  case  no 
trace  of  the  Old  cones  or  the  surface  upon  which  they  were  accumulated 
has  been  preserved,  the  pipes  alone  remain  to  tell  their  tale.  From  the 
fact,  however,  that  these  sometimes  contain  quantities  of  coniferous  wood, 
which  from  its  appearance  must  have  been  buried  in  a  fresh  state,  it  has 
been  inferred  that  some  volcanoes  were  probably  subaerial,  and  that 
after  their  extinction  they  became  clothed  with  a  coniferous  vegetation. 
The  majority  of  the  necks  met  with  in  Scotland,  however,  would  seem 
to  represent  subaqueous  volcanoes.  This  is  suggested  by  the  simple  fact 
that  the  cones  are  occasionally  preserved — which  could  hardly  have 
happened  had  the  volcanoes  erupted  upon  a  land-surface.  The  volcanoes 
referred  to  obviously  discharged  their  ejecta  upon  the  gradually  subsiding 
bed  of  sea,  lagoon,  or  lake,  and  thus  the  sheets  of  materials  that  accumu- 
lated round  the  vents  passed  outwards  in  all  directions  and  became 
interstratified  with  sediments,  charged  with  the  organic  remains  of  the 
period.  When  at  last  the  volcanoes  became  extinct,  they  were  finally 
covered  up  by  successive  deposits  of  sediment,  and  thus  the  cones 
escaped  the  denudation  that  ere  long  must  have  demolished  them  had 
they  been  formed  upon  dry  land  (see  Fig.  72). 


FIG.  72. — CONE  OF  AGGLOMERATE,  AND  NECK  OF  CRYSTALLINE 
IGNEOUS  ROCK. 

The  inward  dip  of  the  strata  surrounding  a  neck  has  been  attributed 
to  that  sinking  of  surface  which  so  frequently  takes  place  near  a  volcanic 
centre.  After  prolonged  activity  the  rocks  surrounding  a  vent  probably 
become  undermined,  and  this  must  tend  to  bring  about  subsidence  in  its 
immediate  neighbourhood.  In  the  case  of  extensive  necks,  from  which 
much  material  has  been  discharged,  the  inward  dip  of  the  surrounding 
strata  may  be  due  to  some  such  cause.  A  large  number  of  the  necks, 
however,  are  too  small  and  erupted  for  too  short  a  period  to  have 
produced  any  marked  subsidence  of  the  surrounding  rock-masses — and 
yet  the  abrupt  inward  dip  of  the  strata  surrounding  such  necks  is  quite  as 


DYKE,  2  FT.  BROAD,  CUTTING  SANDSTONE,  PORT  LEACACH,  ARRAN. 

Photo  by\H.M.  Geological  Survey, 


THE  "YELLOW  MAN."    DYKE  CUTTING  VOLCANIC  AGGLOMERATE,  SHORE  NEAR  NORTH 
^  BERWICK,  HADDINGTONSHIRE. 

Photo  by  H.M.  Geological  Survey. 

[Between  pages  200  and  201. 


STRUCTURE  OF  ERUPTIVE  ROCKS  201 

conspicuous  as  in  the  case  of  larger  ones.  It  would  seem  more  likely  that 
the  sudden  inward  dip  of  the  rocks  abutting  upon  a  neck  is  due  partly  to 
the  downward  drag  of  the  fragmental  materials  while  they  were  slowly 
subsiding  and  becoming  consolidated,  and  partly,  perhaps,  to  the  unequal 
yielding  of  the  strata  themselves  before  the  pipes  were  filled.  When  a 
subaerial  puy  became  extinct,  the  loose  materials  forming  the  cone  would 
naturally  tend  to  slip  down  into  the  crater  and  funnel,  while  at  the  same 
time  the  walls  of  the  vent,  exposed  to  the  action  of  springs  and  to 
weathering  generally,  would  also  supply  material — all  this  debris  falling 
into  the  vent  would  form  a  steep,  rudely-bedded  talus  having  a  centro- 
clinal  dip.  The  falling  away  of  the  softer  and  less  resistant  rocks 
in  the  walls  of  the  vent  would  tend  to  undermine  the  less  yielding  beds 
above  them,  and  thus  cause  these  to  bend  over.  Finally,  when  the  pipe 
had  become  filled,  the  consolidating  debris,  as  it  subsided,  would  drag 
down  the  rocks  forming  the  walls,  and  thus  increase  their  inward  dip. 
In  the  case  of  a  submarine  puy  there  would  be  no  weathering  action 
upon  the  walls  of  the  pipe,  but  it  seems  at  least  unlikely  that  the  rocks 
should  remain  unaffected,  and  that  larger  and  smaller  portions  should  not 
become  detached,  and  thus  cause  undermining  and  bending  downwards 
of  the  rocks  above.  It  is  worthy  of  note,  however,  that  this  inward  dip 
of  the  strata  abutting  against  a  neck  does  not  invariably  occur. 

Necks,  like  batholiths,  may  belong  to  almost  any  geological  period. 
But  inasmuch  as  a  typical  neck  represents  the  upper  portion  of  a  pipe  of 
eruption,  and  consequently  is  not  of  deep-seated  origin,  only  those  of 
subaqueous  eruptions  can  date  back  to  the  earlier  geological  ages.  Now 
and  again,  it  is  true,  some  Palaeozoic  necks  seem  to  have  erupted  on 
land,  but,  if  so,  they  must  ere  long  have  been  submerged,  for  only  in  this 
way  could  these  have  escaped  demolition.  Exposed  for  a  prolonged 
period  to  denudation,  not  only  must  the  cones  have  been  demolished, 
but  the  ancient  land-surface  on  which  these  stood  must  have  been  so 
lowered  that  the  upper  portions  of  the  pipes  of  eruption — the  necks — 
would  have  been  planed  away,  and  the  deeper  seated  roots  of  the  old 
volcanoes  laid  bare. 


CHAPTER    XIV 

ERUPTIVE   ROCKS  :     MODE   OF   THEIR  OCCURRENCE— 
continued 

Dykes  and  Eruptive  Veins — their  General  Phenomena.  Composite 
Dykes.  Exogenous  or  Intrusive  Veins  —  their  association  with 
Batholiths,  etc.  Endogenous  or  Autogenous  Veins.  Pegmatite 
Veins  ;  General  Phenomena  of  Contemporaneous  Veins.  Segrega- 
tion Veins.  Effusive  Eruptive  Rocks — Crystalline  Effusive  Rocks 
and  Pyroclastic  or  Fragmental  Effusive  Rocks. 

4.  DYKES  AND  ERUPTIVE  VEINS 

ERUPTED  matter  which  has  solidified  in  a  more  or  less 
steeply  inclined  or  vertical  and  somewhat  even-sided  fissure, 
is  called  a  dyke,  while  the  term  eruptive  vein  is  usually 
reserved  for  the  more  irregular  and  frequently  tortuous  and 
branching  intrusions.  But  this  usage  is  not  invariable — 
many  geologists  employing  the  terms  interchangeably,  while 
others  designate  as  "  dykes  "  all  the  larger  intrusions,  whether 
wall-like  or  tortuous,  and  restrict  the  term  "  vein "  to  the 
smaller  injections. 

Eruptive  veins  and  dykes  may  consist  of  almost  any  kind 
of  igneous  rock.  Frequently  they  proceed  visibly  from  large 
masses  of  eruptive  rock — bosses  or  sheets,  as  the  case  may  be. 
At  other  times  no  such  relationship  can  be  observed, 
although  we  can  hardly  doubt  that  if  dykes  and  veins  could 
be  followed  downwards  they  would  be  found  to  proceed  in 
the  same  way  from  larger  masses  of  intrusive  character. 

Wall-like  intrusions  are  of  common  occurrence  in  this 
country — the  most  notable  examples  being  the  remarkable 
basalt-dykes  which  are  so  abundantly  developed  in  Central 
and  Western  Scotland  (see  Plate  XL  VI 1 1.).  Sometimes  these 


STRUCTURE  OF  ERUPTIVE  ROCKS  203 

dykes  give  rise  to  conspicuous  surface-features — forming,  as 
the  case  may  be,  either  prominent  ridges  or  elongated 
depressions,  according  as  the  basalt  or  the  rock  it  traverses 
has  offered  the  stouter  resistance  to  denudation.  When  the 
former  is  the  case,  a  dyke  may  rise  wall-like  above  the 
general  level  of  the  country,  continuing  its  course  unin- 
terruptedly for  a  longer  or  shorter  distance  across  hill  and 
dale.  When,  on  the  other  hand,  the  rocks  it  cuts  are  more 
resistant  than  itself,  a  dyke  indicates  its  presence  by  a  long 
narrow  trench  or  depression  instead  of  a  prominent  ridge. 
The  course  followed  by  a  dyke  is,  as  a  rule,  remarkably 
straight  or  direct,  though  often  gently  sinuous.  Occasionally, 
however,  this  regularity  may  be  interrupted  by  one  or  more 
zig-zags  or  sharp  bends.  It  is  noteworthy  that  dykes  which 
traverse  sandstones  and  shales  are  usually  straighter  or  more 
regular  than  those  which  cut  through  greywackes,  crystalline 
igneous  rocks,  and  schists.  While  some  dykes  have  come  up 
along  lines  of  dislocation  or  true  faults — the  great  majority 
occupy  fissures  or  rents  along  which  no  displacement  has 
occurred  (Plates  XLIV.,  XLV.). 

Dykes  vary  in  extent — some  being  considerably  less  than 
a  mile  in  length,  while  others  have  been  followed  for  distances 
of  50  or  even  70  miles  and  more — often  preserving  throughout 
their  course  a  wonderfully  uniform  thickness.  Some  of  the 
smaller  dykes  do  not  seem  to  be  more  than  a  few  inches  in 
thickness — the  longer  ones,  however,  are  much  thicker,  and 
sometimes  reach,  or  even  exceed,  100  feet  in  width.  But 
although  the  shorter  dykes  usually  tend  to  be  thin,  and 
the  longer  ones  to  be  thick,  there  is  really  no  definite 
proportion  between  the  extent  and  the  width  of  dykes  in 
general.  A  dyke  20  feet  thick  may  have  a  longer  range 
than  one  double  its  width — or  the  converse  may  be  the  case. 
Although  no  general  average  can  be  given  for  the  thickness 
of  the  more  persistent  dykes,  yet  it  may  be  said  that  dykes 
measuring  20  to  40  feet  across  are  among  the  commonest 
of  those  which  have  been  followed  for  any  considerable 
distance. 

Occasionally,  a  dyke  divides  into  two  or  more  smaller 
ones — each  pursuing  the  same  general  direction.  Now  and 
again,  also,  eruptive  veins  and  veinlets  proceed  from  a  dyke, 


204 


STRUCTURAL  AND  FIELD  GEOLOGY 


but  this  is  apparently  exceptional.  Dykes  often  wedge  out 
suddenly,  both  in  lateral  and  vertical  directions.  Traced 
across  country,  they  not  infrequently  seem  to  die  out,  and  then 
after  a  shorter  or  longer  interval  they  may  as  suddenly 
reappear.  When  a  dyke  of  this  kind  is  represented  upon  a 
map,  therefore,  we  have  the  appearance  of  two  or  more 
dykes  following  each  other  along  the  same  line.  That  the 
apparently  separate  dykes,  however,  are  really  portions  of 
one  and  the  same  intrusion,  has  now  and  again  been 
demonstrated  in  the  coal-bearing  districts — where  a  dyke 
has  been  followed  continuously  throughout  all  the  coal- 
workings,  although  it  fails  in  some  places  to  reach  the 
surface.  Sometimes,  indeed,  a  dyke  cuts  the  lower  coals  but 
does  not  penetrate  the  higher  seams  in  one  and  the  same 
coal-pit. 

Basalt-dykes  are  jointed  most  prominently  at  right  angles 
to  their  direction — the  jointing  being  frequently  prismatic 
(see  Figs.  73,  74).  But,  in  some  cases,  the  joints  run  parallel 


FIG.  73. — PRISMATIC  JOINTING 
IN  A  DYKE. 


FIG.  74. — COMPLEX  PRISMATIC 
JOINTING  IN  A  DYKE. 


to  the  walls,  so  as  to  give  the  rock  a  kind  of  flaggy  struc- 
ture (see  Plate  XLVI.).  Parallel  jointing  of  this  nature 
is  usually,  however,  confined  to  the  marginal  areas  of  the 
rock. 

The  rock  of  a  dyke  is  almost  invariably  finer  grained 
along  its  margins  than  towards  the  centre — a  structure  which 
is  most  conspicuous  in  the  case  of  the  thicker  dykes.  Thin 
dykes  are  usually  fine-grained  throughout,  yet  even  these 
tend  to  be  most  compact  towards  the  sides.  This  structure 
is  obviously  due  to  the  chilling  effect  of  the  contiguous 


STRUCTURE  OF  ERUPTIVE  ROCKS  205 

rocks — the  dyke  along  the  line  of  junction  becoming  more  or 
less  markedly  vitreous.  Small  vapour  pores  often  appear  at 
or  near  the  margin,  while  larger  pores,  vacuoles,  and  occasion- 
ally irregular  shaped  cavities  of  some  size  occur  towards  the 
centre,  either  sporadically  or  forming  a  continuous  medial 
zone  running  parallel  to  the  direction  of 
the  dyke  (Fig.  75). 

Dykes  affect  the  contiguous  rocks 
much  in  the  same  way  as  sheets,  but  to 
a  less  extent.  In  the  case  of  dykes  only 
a  few  feet  in  thickness,  the  alteration 
produced  is  very  slight,  but  the  broader 
dykes  may  bake  and  indurate  the  rocks 
for  a  yard  or  two  away. 

Occasionally   a   dyke   is   the   product 
of   more   than   one   intrusion — the   same 
fissure  having  been  rent  open  again  and     FIG.  75.— DYKE,  snow- 
again  so  as  to  allow  of  successive  injec-        ING  USUAL  POSITION 
.s  J  OF   VAPOUR    PORES 

tions  of  the  same  kind  of  molten  matter        AND  VESICLES. 

— the  younger  injections  being  often 
readily  recognised  by  the  "  chilled  edges  "  which  they  present 
to  the  rock  they  traverse.  In  other  cases,  however,  the  earlier 
and  the  later  injections  may  be  distinctly  different — an  erup- 
tion of  basic  rock  having  either  preceded  or  succeeded  one  of 
acid  rock.  In  such  composite  dykes  a  clear  line  of  demarcation 
separates  one  injection  from  another.  But  in  certain  broad 
dykes  of  a  composite  structure,  no  such  lines  of  separation  are 
visible — one  kind  of  rock  gradually  merging  into  another,  so 
that  the  whole  complex  must  obviously  have  been  injected 
at  or  about  the  same  time.  The  rock  forming  the  sides  of  a 
dyke  of  this  character  is  usually  more  basic  than  the  central 
and  larger  portion.  Near  Liebenstein,  in  the  Thuringian 
Forest,  for  example,  there  is  a  broad  dyke,  the  flanks  of  which 
consist  externally  of  melaphyre,  which  graduates  inwards  into 
syenite-porphyry,  as  this  in  turn  merges  into  granite-porphyry, 
of  which  the  central  and  major  mass  of  the  dyke  is  composed 
(see  Fig.  76).  Dykes  of  a  like  kind  have  been  described  by 
Professor  A.  C.  Lawson  as  occurring  in  the  Rainy  Lake 
region  of  Canada — where  in  one  and  the  same  dyke  the 
andesite  of  the  marginal  areas  shades  off  inwards  into  a 


206 


STRUCTURAL  AND  FIELD  GEOLOGY 


central  quartz-gabbro.      Phenomena  of  this  kind  are  doubt- 
less due  to  magmatic  differentiation. 

Eruptive    veins    and   dykes,   as   already   indicated,   often 
follow   somewhat  erratic  courses.     The  more  or  less  regular 


fp^y^^l^- 

fe^-rm- 

•"• 


FIG.  76.— COMPOSITE  DYKE,  LIEBENSTEIN  (THURINGIA).    (After  Dr 

K.  Keilhack). 
a,   a,  granite ;    m,  in,  melaphyre ;    s,   s,   syenite-porphyry ;    G-p,  granite-porphyry. 

basalt-dykes  of  Central  Scotland  have  been  cited  as  good 
examples  of  wall-like  intrusions.  It  need  hardly  be  said, 
however,  that  injections  of  basalt,  as  of  any  other  kind  of 
igneous  rock,  are  often  extremely  tortuous  and  branching  (see 
Plate  XLVI.).  The  veins  usually  associated  with  granite, 
however,  may  be  taken  as  somewhat  characteristic  of  their 
kind.  Of  these  veins  two  types  are  recognised — exogenous  or 
intrusive  and  autogenous  or  endogenous  veins. 


FIG.    77. — VEINS   PROCEEDING   FROM    A   MASS   OF   GRANITE. 

Exogenous  or  Intrusive  Veins. — These  are  simply  protrusions  pro- 
ceeding from  a  mass  of  granite  into  the  contiguous  rocks.  They  vary  in 
thickness  from  mere  lines  or  threads  up  to  many  feet  or  yards.  Usually 
very  tortuous,  they  ramify  in  all  directions,  intercrossing,  dividing  and 
subdividing  again  and  again.  Now  and  again,  extremely  thin  veins 


STRUCTURE  OF  ERUPTIVE  ROCKS  207 

have  forced  their  way  along  planes  of  cleavage  or  of  foliation.  As  a  rule, 
however,  small  and  large  veins  alike  follow  no  definite  direction — -save 
that  they  stream  outwards  from  the  margin  of  the  parent  batholith — 
gradually  diminishing  in  numbers  as  they  proceed.  In  many  cases  the 
veins  form  a  perfect  network  amongst  which  irregular  fragments  and 
larger  masses  of  the  invaded  rocks  appear  as  if  entangled— forming  what 
is  termed  an  injection  plexus.  All  the  phenomena,  indeed,  seem  to  sug- 
gest that  before  the  veins  were  injected  the  rocks  surrounding  a  batholith 
had  been  so  profoundly  shattered,  that  molten  matter  found  little  diffi- 
culty in  making  its  way  amongst  the  fractured  and  sundered  masses 
(Fig.  77).* 

The  rock  of  these  veins,  especially  the  smaller  ones,  is  usually  finer 
grained  than  the  granite-mass  from  which  it  comes.  It  is  notable,  also, 
that  not  infrequently  it  differs  in  petrographical  character  from  that  of 
the  parent-rock — many  of  the  veins  consisting  of  quartz-porphyry  or 
felsite. 

Endogenous  or  Autogenous  Veins. — Some  of  these  are  composed  of 
finer  grained  rock  than  the  granite,  and  usually  differ  from  it  in  being 
more  acid.  Others,  again,  are  characterised  by  the  intergrowth  of  the 
constituent  quartz  and  felspar.  These  are  the  pegmatite-veins.  They 
are  generally  coarser  grained  than  the  rock  they  traverse.  The  precise 
mode  of  origin  of  these  endogenous  veins  is  quite  uncertain.  Although 
obviously  younger  than  the  rock  they  cut,  they  yet  appear  to  form  portions 
of  the  same  intrusive  mass— to  be  merely  modifications,  as  it  were,  of  the 
granite  itself.  Hence  they  are  often  spoken  of  as  contemporaneous  veins. 
They  are  supposed  to  belong  to  the  period  of  cooling  and  consolidation,  and 
to  have  been  injected  from  still  liquid  portions  of  the  magma  into  rents 
formed  during  movements  of  the  surrounding  solidified  or  partially  solidified 
mass.  This  seems  a  plausible  explanation  of  the  fine-grained  autogenous 
veins,  but  it  does  not  account  for  the  structure  of  the  coarsely  crystalline 
pegmatite  veins.t  The  contemporaneous  origin  of  both  fine-grained  and 

*  The  exploitation  of  "contact  ore-formations"  (see  Chapter  XVII.) 
has  shown  that  the  ore-bearing  rocks  overlying  and  surrounding  a 
plutonic  mass  are  often  much  jumbled  and  shattered — shales  and  lime- 
stones, for  example,  being  converted  into  breccias  which  are  usually 
highly  silicified.  These  brecciated  masses  may  occur  at  a  considerable 
distance  from  the  intrusive  rock,  and  possibly  owe  their  origin  to  the 
explosive  action  of  steam  and  vapours.  Not  infrequently  they  are 
traversed  by  dykes  and  eruptive  veins,  but  these  could  not  have  caused 
the  shattering  of  the  rocks,  for  the  same  dykes  cut  through  undisturbed 
areas  where  no  brecciation  is  visible. 

t  According  to  Professor  Arrhenius,  a  granite  magma  containing 
sufficient  water  would,  in  cooling,  probably  separate  into  two  portions — the 
product  of  the  separation  appearing  as  an  aqueous  solution  in  which  would 
be  concentrated  such  bodies  as  are  more  soluble  in  water  than  in  the  silicate 
magma.  Owing  to  their  greater  mobility  than  the  magma,  these  aqueous 


208  STRUCTURAL  AND  FIELD  GEOLOGY 

TV**     '.  '.'*?"  *  -»* .  ^^ 

coarse-grained  autogenous  veins  is  shown  by  the  fact  that  they  are  not 
always  sharply  separated  from  the  rock  on  either  side,  as  is  the  case  with 
exogenous  or  intrusive  veins.  On  the  contrary,  the  mineral  constituents 
of  an  autogenous  vein  often  interosculate,  as  it  were,  with  those  of  the 
surrounding  granite — the  crystals  of  the  latter  being  so  interlocked  with 
those  of  the  vein,  that  the  two  rocks  are  not  readily  separated  along  their 
line  of  junction. 

Contemporaneous  veins  are  met  with  in  many  other  eruptive  rocks, 
more  particularly  in  batholiths  and  thick  sills  of  such  rocks  as  gabbro, 
dolerite,  and  diorite. 

Yet  another  kind  of  autogenous  veins  may  be  mentioned.  These 
are  the  so-called  segregation  veins.  They  are  distinguished  from  the 
other  varieties  described  by  the  fact  that  they  merge  gradually  into  the 
enclosing  rock  of  which,  therefore,  they  are  merely  a  coarsely-crystalline 
modification.  They  have  not  been  injected  into  rents  or  fissures  after 
the  manner  of  other  endogenous  veins,  but  their  precise  mode  of  origin 
is  obscure.  They  appear  to  be  the  result  of  some  process  of  segregation, 
and  to  represent  zones  or  lines  along  which  crystallisation  of  the  con- 
stituent minerals  was  more  readily  developed  than  elsewhere  in  the 
same  rock-mass.  Although  of  common  occurrence  in  eruptive  rocks, 
segregation-veins  are  not  confined  to  these,  but  make  their  appearance 
also  in  certain  schists,  and  even  in  derivative  rocks  which  have  been 
more  or  less  metamorphosed. 

EFFUSIVE  ERUPTIVE  ROCKS 

Effusive  rocks  have  been  erupted  at  the  earth's  surface, 
and  are  of  two  types,  crystalline  and  fragmental — that  is  to 
say,  lavas  and  tuffs.  As  they  frequently  occur  interstratified 
in  a  conformable  manner  with  derivative  rocks  of  all  kinds, 
they  are  often  termed  contemporaneous  or  interbedded. 

(a]  Crystalline  Effusive  Rocks. — The  general  petro- 
graphical  characters  of  these  rocks  have  been  already  set 
forth.  It  will  be  remembered  that  lavas  are  often  scoriaceous 
above  and  below,  and  in  some  cases  may  be  more  or  less 
porous  and  cavernous  throughout.  The  vapour-cavities  are 
often  flattened  or  drawn-out  in  the  direction  of  flow.  In  all 
such  lava-form  rocks  residual  glassy  matter  is  very  commonly 
present,  especially  towards  the  upper  and  under  surfaces. 

solutions  might  send  out  the  very  finest  threads  and  veins  into  the 
contiguous  rocks,  while  other  portions  would  collect  as  geodes  and  veins 
in  the  interior  of  the  magmatic  mass.  As  the  solution  cooled,  one 
substance  after  another  would  separate  out — and  if  the  cooling  process 
were  not  too  rapid,  the  minerals  would  segregate  in  large  crystals,  such  as 
characterise  the  pegmatite-veins. 


[To  face  page  208. 


STRUCTURE  OF  ERUPTIVE  ROCKS  209 

The  mineral  constituents  also  frequently  show  glass-  and 
stone-inclusions,  while  liquid-cavities  are  relatively  seldom 
seen.  Now  and  again  the  lower  part  of  a  lava  is  crowded 
with  indurated  arenaceous  and  argillaceous  matter,  and 
contains  occasionally  well  water-worn  stones,  as  if  the  molten 
matter  had  flowed  over  the  bed  of  the  sea  or  of  a  lake  or 
river,  and  thus  caught  up  and  enclosed  some  of  the  sedimentary 
materials  lying  in  its  path.  Even  fragments  of  trees  have 
been  found  included  in  the  basal  portion  of  a  lava — as  in  the 
case  of  a  Carboniferous  basalt-flow  near  Kinghorn,  Fife.  In 
all  these  respects  effusive  crystalline  rocks  differ  markedly 
from  intrusive  rocks.  As  further  differentiating  lava-form 
rocks  from  sills,  with  which  they  might  sometimes  be  con- 
founded, it  may  be  noted  that  while  the  former  may  produce 
some  induration  of  the  rocks  on  which  they  rest,  they  never 
affect  the  overlying  strata.  Obviously,  the  superjacent  beds 
have  been  deposited  over  the  surface  of  the  lava-form  rock 
after  consolidation  had  taken  place,  for  the  lines  of  bedding 
follow  all  the  irregularities  of  the  underlying  rock-surface. 
When  this  is  much  rent  and  cleft,  the  cavities  have  been 
gradually  filled  up  with  sediment,  while  now  and  again 


FIG.  78.— EFFUSIVE  IGNEOUS  ROCKS. 

I,  I,  lava-flows ;  t,  tuflaceous  sandstones  ;  fi,  tuffaceous  shales. 

fragments  of  the  scoriaceous  crust  of  the  old  lava  have  been 
detached  and  enclosed  in  the  immediately  superjacent  aqueous 
rock.  Again,  it  may  be  noted  that  lava-form  rocks  are 
usually  associated  with  stratified  tuffs  (see  Fig.  78). 

O 


210  STRUCTURAL  AND  FIELD  GEOLOGY 

Flows  vary  in  thickness,  some  being  only  a  few  feet, 
while  others  attain  a  depth  of  many  yards.  The  more  basic 
lavas  generally  preserve  a  somewhat  equable  thickness,  the 
intermediate  and  acid  kinds  tending  rather  to  be  irregular, 
so  that  they  thicken  and  thin-out  more  or  less  rapidly. 

(&)  Pyroclastic  or  Fragmental  Effusive  Rocks.— The 
tuffs  usually  associated  with  lava-form  rocks  vary  in  character. 
As  might  have  been  expected,  their  dominant  ingredients 
consist  of  the  comminuted  debris  and  larger  fragments  of 
the  lavas  they  accompany.  Thus  we  have  basalt-tuff, 
andesite-tuff,  trachyte-tuff,  etc.  All  varieties  of  texture  and 
structure  are  met  with,  some  rocks  being  very  fine  grained, 
while  others  are  mere  aggregates  of  lapilli  and  blocks — finer 
and  coarser  grained  materials  often  rapidly  alternating  in 
a  vertical  section.  Bedding  is  usually  pronounced — many 
of  the  finer  tuffs  being  beautifully  laminated.  Occasionally, 
very  large  sporadic  blocks  may  be  encountered  in  a  bedded 
mass  of  small  lapilli,  and  generally  increase  in  numbers 
as  the  old  focus  of  eruption  is  approached.  Tuffs  are  fre- 
quently interstratified  with  ordinary  sedimentary  beds,  and 
when  such  is  the  case  the  tuffs  themselves  usually  contain 
a  larger  or  smaller  proportion  of  arenaceous  or  argillaceous 
materials,  and  thus  frequently  graduate  into  sandstone  and 
shale.  Fossils  may  be  included  not  only  in  the  sedimentary 
beds  associated  with  tuffs,  but  in  the  tuffs  themselves. 
Fragments  of  plants,  and  various  marine  organic  remains, 
for  example,  not  infrequently  occur  in  the  tuffs  and  tuffaceous 
sandstones  and  shales,  which  are  associated  with  the  andesitic 
lavas  of  the  Carboniferous  system  in  Scotland. 

Mode  of  Occurrence  of  Effusive  Rocks. — Sometimes  a 
flow,  with  its  accompanying  tuff,  occurs  singly ;  more  usually, 
however,  flows  and  tuffs  appear  in  consecutive  series.  Some 
effusive  rocks,  occupying  a  limited  area,  are  obviously  the 
products  of  an ,  isolated  volcano.  Others  extend  over  very 
wide  regions,  and  appear  to  represent  the  products  of  a  series 
of  more  or  less  closely  associated  foci  of  eruption,  the 
successive  lavas  and  tuffs  discharged  from  the  several  vents 
interosculating  and  overlapping.  A  good  example  is  furnished 
by  the  eruptive  rocks  of  the  Sidlaw  and  Ochil  Hills,  some 
of  the  old  vents  from  which  these  were  discharged  being 


STRUCTURE  OF  ERUPTIVE  ROCKS  211 

still  recognisable  in  the  great  necks  and  bosses  which  have 
been  exposed  by  denudation.  In  other  cases  of  widely 
extended  effusive  rocks,  we  appear  to  have  the  products  of 
vast  fissure-eruptions.  Of  such  a  character  are  the  plateau- 
basalts  of  the  Western  Islands  of  Scotland,  Antrim,  the 
Faeroe  Islands,  Iceland,  etc.  At  the  time  of  these  eruptions, 
the  whole  wide  region  extending  from  the  British  Islands  to 
Greenland  appears  to  have  been  underlaid  by  a  vast  sea  of 
molten  matter,  which  rose  to  the  surface  along  rents  in  the 
crust  and  deluged  the  surrounding  areas  with  floods  of  lava. 
Such  rents  and  fissures  were  doubtless  the  result  of  earth- 
quake action ;  and  many  of  them  did  not  reach  the  surface, 
dying-out  upwards  at  various  levels.  Into  these,  however, 
molten  matter  found  its  way,  forming  the  great  series  of 
basalt-dykes  shown  in  the  map,  Plate  XLVIIL* 

Sandstone  Dykes. — Here  brief  reference  may  be  made  to  certain 
abnormal  dykes,  occurring  in  California  and  elsewhere  in  North  America. 
They  are  composed  of  sedimentary  materials,  and  occupy  vertical  fissures, 
which  have  been  filled  not  from  above  but  from  below.  Some  of  these 
dykes  have  a  length  of  several  miles,  and  their  precise  mode  of  origin  is 
obscure.  The  sand  may  have  been  introduced  from  below  during  earth- 
quake movements.  For  unconsolidated  materials,  such  as  water-logged 
clay  and  sand,  when  buried  under  a  considerable  thickness  of  super- 
incumbent rock,  are  ready  to  rise  towards  the  surface  along  any  open 
fissures  that  may  be  formed.  Occasionally  boring  operations  in  our  coal- 
fields have  been  impeded  in  this  way  by  the  more  or  less  rapid  rising  in  a 
bore-hole  of  soft  clay,  coming  from  a  considerable  depth. 

*  It  ought  to  be  mentioned,  however,  that  some  of  the  dykes  shown 
upon  the  map  date  back  to  much  earlier  periods.  For  example,  certain 
dykes  traversing  the  Carboniferous  tracts  appear  to  be  of  late  Carboni- 
ferous age. 


CHAPTER   XV 

ALTERATION   AND    METAMORPHISM 

Rock-changes  induced  by  Epigene  Action.  Deep-seated  Alteration  or 
Metamorphism.  Degrees  of  Metamorphism.  Thermal  or  Contact 
Metamorphism.  Regional  Metamorphism  —  Plutonic,  Hydro- 
chemical,  and  Dynamo-metamorphism. 

Alteration  by  Epigene  Action. — Very  few  rocks  have  not 
undergone  some  change  since  the  time  of  their  formation. 
At  and  for  some  distance  down  from  the  surface  water  passes 
more  or  less  readily  along  the  various  planes  of  division  by 
which  all  rocks  are  traversed — not  only  so,  but  it  soaks  into 
the  rocks  themselves,  occupying  their  minutest  pores  and 
capillaries.  In  this  way  chemical  changes  of  greater  or  less 
importance  are  effected,  by  which  certain  rocks  tend  to 
become  disintegrated,  while  others,  on  the  contrary,  are  more 
firmly  consolidated.  Crystalline  igneous  rocks,  as  a  rule,  are 
prone  to  decay — their  felspathic  and  ferromagnesian  con- 
stituents being  readily  broken  up  chemically,  and  some 
portion  of  their  substance  removed  in  solution.  Many 
schistose  rocks  experience  the  same  kind  of  change — a 
change  which  usually  results  in  weakening  a  rock — its 
hardness  and  solidity  becoming  more  or  less  impaired. 
Sedimentary  rocks,  on  the  other  hand,  being  themselves  the 
products  of  decay  and  disintegration,  and  consisting  there- 
fore of  more  stable  ingredients,  are  less  liable  to  those 
chemical  changes  to  which  igneous  and  schistose  rocks  alike 
are  subject.  Instead  of  being  weakened  by  the  action  of 
percolating  water,  they  are  often  strengthened  by  the  intro- 
duction into  their  pores  and  capillaries  of  various  mineral 
substances  which  bind  their  ingredients  more  firmly  together. 
To  this  general  rule  there  are,  as  might  have  been  expected, 


212 


ALTERATION  AND  METAMORPHISM  213 

many  exceptions.  Percolating  water,  which  introduces 
cementing  materials,  may  in  the  course  of  time  redissolve 
these  and  carry  them  away.  Again,  rocks  of  chemical 
origin,  such  as  travertine,  dolomite,  etc.,  and  rocks  organically 
derived,  such  as  chalk  and  many  limestones,  being  all  more  or 
less  soluble,  are  readily  attacked  by  percolating  water.  To 
sum  up  in  a  few  words,  it  may  be  said  that  the  chief  chemical 
changes  induced  in  rocks  by  the  process  of  weathering, 
consist  of  solution,  oxydation,  hydration,  and  the  formation 
of  carbonates  and  sulphates. 

Metamorphism. — The  changes  brought  about  by  epigene 
action,  however  extreme  they  may  be,  must  not  be  confounded 
with  true  metamorphism.  The  term  "  metamorphic  "  is  applied 
properly  to  rocks,  the  texture,  structure,  and  mineralogical 
constitution  of  which  have  been  more  or  less  profoundly 
affected.  Metamorphism,  however,  varies  much  in  its 
intensity.  It  may  be  so  inconsiderable  as  not  to  obscure 
all  original  characters,  or  so  extreme  that  we  can  only 
conjecture  what  the  nature  of  the  unaltered  rock  may  have 
been.  True  metamorphism,  especially  that  which  has 
resulted  in  crystallisation  and  recrystallisation  and  the  pro- 
duction of  foliation,  would  seem  to  have  taken  place  at  some 
depth  from  the  surface,  and  to  have  been  induced  proximately 
by  heat,  usually  if  not  always  in  the  presence  of  water  or 
vapours.  Metamorphic  rocks  have  a  certain  aspect  which 
commonly  serves  to  distinguish  them  from  rocks  altered  by 
epigene  action  alone.  The  great  majority  are  more  or  less 
indurated,  crystalline,  or  subcrystalline,  and  foliated  or 
schistose.  Seldom,  indeed,  can  an  igneous  or  derivative  rock 
altered  by  epigene  action  be  mistaken  for  a  metamorphic 
rock.  Nevertheless,  there  are  certain  altered  rocks  which  in 
hand-specimens  might  quite  well  pass  for  products  of 
metamorphism.  Sands  and  sandstones,  for  example,  have 
frequently  been  transformed  into  quartzite  by  percolating 
water  carrying  silica  in  solution ;  and  hand-specimens  of 
such  rocks  might  readily  be  taken  for  quartzites  of  truly 
metamorphic  origin.  Serpentine  affords  another  example  of 
a  rock  which  has  resulted  sometimes  from  epigene  and 
sometimes  from  hypogene  action.  Metamorphic  serpentine, 
however,  is  usually  foliated,  and,  moreover,  is  always  associated 


214  STRUCTURAL  AND  FIELD  GEOLOGY 

with  other  crystalline  schistose  rocks.  On  the  other  hand, 
serpentine  of  epigene  origin  is  not  foliated,  and  is  found 
traversing  rocks  of  all  kinds,  while  its  igneous  character  can 
readily  be  determined  by  field  observation.  Cases  like  these, 
however,  are  exceptional,  and  there  is  usually  no  difficulty  in 
distinguishing  in  the  field  between  metamorphic  rocks  and 
rocks  which  have  been  altered  by  epigene  action. 

There   are    many   degrees   of  metamorphism.      In  some 
cases,  rocks  have  been  so  slightly  changed  that  their  distinctive 
characters  have  remained  unaffected.     Reference  has  already 
been  made  to  the  transformation  of  a  relatively  soft  quartzose 
sandstone  into  a  hard  quartzite.     Here  the  only  conspicuous 
change    is   one   of  texture :    while   becoming   indurated,   the 
original    rock   has   retained    its    chemical    composition    and 
structure.     Planes  of  stratification,  diagonal  or  cross-bedding, 
ripple-marks,  etc.,  may  be  as  conspicuous  in  a  quartzite  as  in 
any  unaltered  sandstone.     In    most  cases,  however,  a   rock, 
while   it    retains   its   chemical   composition   unchanged    after 
metamorphism,  has  yet  been  profoundly  modified  as  regards 
its    constitution    and    structure.      An    argillaceous   shale,    for 
example,  may  be  transformed  into  an  andalusite-mica-schist, 
without  either  loss  or  gain  of  mineral  substance.     Similarly, 
eruptive  rocks,  such  as  granite,  gabbro,  diorite,  etc.,  may  be 
rendered   schistose— the   ultimate    chemical    composition    of 
each  remaining  practically  unchanged.     Nor  is  foliation  the 
only   modification   induced    in   eruptive    masses — for    one  or 
other   of  their  essential  constituents   may  be   transformed — 
pyroxene,  for  example,  has  often  been  changed  into  amphibole. 
Thus,  dolerite  has    not   infrequently   been   transformed    into 
hornblende-schist       Although   the   chemical   composition    of 
rocks  has  not  usually  been  much  affected  by  metamorphism, 
yet  this  is  not  invariably  the  case.     Occasionally,  there  has 
been  a  loss  of  mineral  substance — volatile  elements,  such  as 
carbon-dioxide   and   water,   having    been   driven   out — more 
frequently,  however,  the  opposite  has  been  the  case,  and  new 
materials  (silica,  alkalies,  fluorine,  etc.)  have  been  introduced. 

Two  phases  of  metamorphism  are  recognised,  namely, 
(a)  thermal  or  contact  metamorphism,  and  (b)  regional  or 
general  metamorphism. 

(a)  Thermal  or  Contact  Metamorphism. — Reference  has 


ALTERATION  AND  METAMORPHISM  215 

already  been  made  to  the  fact  that  rocks  which  have  been 
overflowed  or  invaded  by  molten  matter  are  usually  more  or 
less  altered  along  the  line  of  contact.  The  changes  effected 
by  a  lava-stream  are  not  particularly  conspicuous,  and  consist 
chiefly  of  induration,  often  accompanied  in  the  case  of  clay 
by  a  change  of  colour  and  the  production  of  prismatic  joint- 
ing. The  changes  caused  by  intrusive  eruptive  rocks, 
however,  are  usually  more  pronounced.  Sometimes,  indeed, 
they  are  of  slight  importance  and  confined  to  the  immediate 
proximity  of  the  intrusion ;  but  at  other  times  they  may 
extend  outwards  from  the  margin  of  the  eruptive  mass  for 
hundreds  or  thousands  of  yards.  The  extent  and  intensity 
of  the  metamorphism  depend  partly  upon  the  character  and 
mass  or  volume  of  the  intruded  rock,  and  partly  upon  the 
nature  of  the  rocks  invaded.  Other  things  being  equal,  more 
change  is  effected  by  an  extensive  eruptive  mass  than  by  a 
smaller  intrusion  of  the  same  kind  of  rock,  while  certain  rocks, 
owing  to  their  composition,  are  more  readily  influenced  than 
others. 

Some  reference  has  already  been  made  to  the  kind  of  changes 
produced  upon  contiguous  strata  by  basic  intrusive  rocks — such  as  the 
conversion  of  coal  into  coke,  anthracite,  or  graphite,  the  crystallisation  of 
limestone,  the  induration  of  rocks  generally,  the  production  of  prismatic 
jointing,  etc.  These  and  other  changes  are  often  exhibited  by  the 
larger  and  smaller  fragments  of  sandstone,  shale,  etc.,  which  have  been 
torn  from  their  parent  strata  and  enclosed  in  an  eruptive  rock.  The 
larger  included  slabs  and  blocks  are  usually  much  shattered,  baked, 
corroded  or  fused  superficially,  and  even  occasionally  rendered  vesicular 
or  scoriaceous.  Pieces  of  felspathic  sandstone  have  been  thoroughly 
fused,  while  fragments  of  dark  shale  have  been  burnt  red  and  baked 
into  a  hard  porcellanite.  Similarly,  when  basalt  has  caught  up  and 
enclosed  portions  of  some  igneous  or  schistose  rock,  such  as  granite  or 
gneiss,  these  have  been  either  partially  or  completely  fused  to  a  dark 
green  or  black  glass.  It  is  noteworthy  that,  in  the  fused  portions  of 
such  included  blocks  and  fragments  of  various  kinds  of  rock,  new 
minerals  (cordierite,  spinel,  sillimanite,  pyroxene,  etc.)  have  not  infre- 
quently been  developed. 

Similar  changes  are  effected  on  the  rocks  in  situ  along  their  line  of 
contact  with  sills  and  dykes  of  basalt  or  other  basic  igneous  rock.  Fusion, 
however,  is  confined  to  the  actual  line  of  contact,  while  induration  and 
other  changes  may  extend  outwards  for  many  feet  or  yards,  the  width  of 
the  metamorphosed  belt  being  dependent  on  the  volume  of  the  eruptive 
mass,  and  to  a  large  degree  also  upon  the  character  of  the  surrounding 


216 


STRUCTURAL  AND  FIELD  GEOLOGY 


rocks.  Thus  coals  may  become  coked  at  a  distance  of  many  yards  from 
a  basalt,  while  the  intervening  sandstones  and  shales  may  show  little  or 
no  change  beyond  slight  induration  or  discoloration.  Limestone  is 
likewise  somewhat  readily  influenced  by  basalt — the  rock  becoming 
converted  into  a  crystalline  marble,  for  a  few  feet  or  more  from  the  line 
of  contact. 

But  the  most  notable  contact  metamorphism  is  induced  by  great 
plutonic  batholiths — more  especially  by  granite.  The  phenomena  are 
perhaps  most  conspicuously  displayed  in  places  where  the  rocks  surround- 
ing a  granite  consist  of  what  were  originally  more  or  less  unaltered 
greywacke  and  shale,  or  other  strata  of  derivative  origin.  In  such  a 
region  one  can  study  all  the  various  modifications  which  the  strata 
undergo,  as  they  are  followed  towards  their  contact  with  the  eruptive 
mass.  The  zone  or  aureole  of  altered  rocks  surrounding  a  large  batholith 
of  granite  may  be  a  mile  or  more  in  width  (see  Fig.  79).  Along  the 


FIG.  79. — BATHOLITH  WITH  AUREOLE  OF  METAMORPHOSED  ROCKS. 

g,  granite ;  gw,  greywackes  and  shales  ;  m,  metamorphosed  rocks. 

outer  margin  of  this  zone,  clastic  rocks  begin  to  show  more  or  less  notable 
evidence  of  induration.  In  these  indurated  but  otherwise  unaltered  rocks, 
the  changes  produced  depend  largely,  as  we  have  seen,  on  their 
mineralogical  and  chemical  composition.  Should  the  rocks  be  essentially 
argillaceous,  aluminous  silicates,  such  as  chiastolite,  often  make  their 
appearance,  while  at  the  same  time  biotite  may  be  developed.  Occa- 
sionally, when  carbonaceous  matter  is  diffused  through  a  shale  or  slate 
this  may  become  aggregated  to  form  more  or  less  abundant  dark  spots, 
and  so  give  rise  to  one  type  of  the  rock  known  as  spotted  slate  (see  Plate 
XXII.  i).  These  carbonaceous  spots  disappear  as  metamorphic  change 
increases  and  a  schistose  structure  is  superinduced.  In  other  cases  the 
spots  take  the  form  of  concretionary  knots,  which  seem  to  consist 
essentially  of  micaceous  matter,  cordierite,  or  andalusite.  Knotted  or 
spotted  slates  of  this  kind  usually  contain  other  new  minerals,  such  as 


ALTERATION  AND  METAMORPHISM  217 

quartz  and  mica,  and  as  metamorphism  advances,  schistosity  becomes 
more  and  more  pronounced — the  foliation  being  developed  along  pre- 
existing planes  of  division,  as  bedding  or  cleavage.  Such  a  rock  thus 
gradually  merges  into  mica-schist  or  andalusite-mica-schist,  often 
containing  cordierite.  This  schistose  rock  in  its  turn  eventually  may 
become  transformed,  in  close  proximity  to  the  batholith,  into  an  exceed- 
ingly hard  compact  hornfels,  in  which  no  trace  of  schistosity  may  be 
observed. 

In  the  aureole  surrounding  a  batholith  greywacke  may  undergo 
similar  changes.  Knots  may  be  developed  in  them,  and  if  the  original 
rocks  contained  much  felspathic  matter,  they  may  be  transformed  into 
rudely  foliated  or  gneiss-like  mica-quartz-rock,  with  cordierite  in  less  or 
greater  abundance.  The  metamorphism  of  sedimentary  rocks  being 
dependant  on  their  chemical  character  it  is  obvious  that  the  succession 
of  changes  witnessed  in  the  neighbourhood  of  intrusive  masses  must 
vary  with  the  varying  nature  of  the  surrounding  rocks.  Limestone, 
for  example,  is  transformed  into  marble,  through  which  new  minerals 
are  disseminated,  such  as  tremolite,  lime-garnet,  idocrase,  zoisite,  and 
other  lime-silicates.  These  new  minerals  doubtless  represent  the 
impurities  (sand,  clay)  diffused  through  the  original  unaltered  limestone. 
When  they  are  very  abundant  the  rock  passes  into  calc-silicate  hornfels. 
Pure  siliceous  sandy  rocks  are  changed  into  quartzites  and  hard 
jaspideous  schists  ;  but  should  the  original  unaltered  rocks  have  con- 
tained argillaceous  matter,  this  is  sure  to  be  represented  by  the 
development  of  new  minerals,  such  as  mica.  The  molecular  rearrange- 
ment of  rock-ingredients  and  the  chemical  recombinations  which 
result  in  the  production  of  "new"  minerals  is  one  of  the  most  notable 
phenomena  of  thermal  and  regional  metamorphism  alike.  Equally  note- 
worthy is  the  appearance  of  schistosity,  which  so  frequently  accompanies 
extreme  rock-change.  In  thermal  metamorphism,  however,  this  structure 
is  usually  met  with  only  in  the  immediate  neighbourhood  of  a  batholith, 
and  it  is  not  always  present — the  rocks  in  contact  with  an  eruptive  mass 
often  appearing  as  highly  compact,  fine-grained,  or  coarsely  crystalline 
rocks  ("hornfelses")  without  any  trace  of  foliation. 

Schists  and  even  igneous  rocks,  when  they  are  traversed  by  batholiths, 
become  metamorphosed,  but  the  changes  induced  are  less  striking,  and 
consist  chiefly  of  recrystallisation  and  structural  modifications.  Schists, 
for  example,  may  become  highly  contorted  and  puckered  as  they  approach 
a  batholith.  Igneous  rocks,  likewise,  are  affected  by  plutonic  intrusions 
—rearrangements  and  recombinations  of  their  ingredients  taking  place, 
changes  which  are  usually  accompanied  by  the  development  of  new 
minerals. 

Not  only  are  rocks  of  all  kinds  more  or  less  metamorphosed  by 
intrusive  masses,  but  the  igneous  masses  themselves  are  not  infrequently 
affected  by  the  rocks  amongst  which  they  have  been  intruded.  Some 
remarkable  examples  have  been  cited  by  French  geologists.  In  the 
Pyrenees,  for  instance,  normal  granite  in  contact  with  calcareous  strata 
becomes  hornblendic  and  passes  into  diorite,  which  may  or  may  not 


218  STRUCTURAL  AND  FIELD  GEOLOGY 

contain  quartz.  In  other  places,  where  the  surrounding  rocks  are  not 
calcareous,  the  same  granite  is  transformed  into  rocks  of  a  still  more 
basic  character,  such  as  norites  and  peridotites.  Numerous  xenoliths 
are  scattered  through  the  granite — all  being  metamorphosed  and  often 
passing  by  insensible  gradations  into  the  igneous  mass  that  surrounds 
them. 

It  is  believed  that  water  has  played  an  important  role  in  thermal 
metamorphism.  Deep-seated  magmas  probably  contain  large  supplies 
of  water  and  other  vapours  and  gasses  dissolved  in  them,  the  presence 
of  which  must  increase  the  liquidity  of  the  molten  masses.  Indeed, 
direct  evidence  of  the  existence  of  this  contained  water  is  supplied  by 
volcanic  phenomena,  vast  volumes  of  steam  and  vapours  issuing  from 
craters  and  escaping  from  lavas.  Unaltered  sedimentary  rocks  also  con- 
tain considerable  stores  of  water,  for  all  are  more  or  less  porous,  and  are 
thus  capable  of  retaining  a  larger  or  smaller  proportion  of  interstitial 
moisture.  In  addition  to  this  supply  we  must  take  note  of  the  fact  that 
many  of  the  mineral  constituents  of  rocks  contain  water  in  chemical 
combination.  It  is  not  surprising,  therefore,  that  the  more  important 
metamorphic  changes  effected  by  a  batholith  are  just  such  as  should 
have  been  produced  by  steam  permeating  the  rocks  under  great  pressure 
and  at  a  very  high  temperature.  The  steam  has  simply  acted  as  a  solvent 
menstruum,  and  has  tended  to  produce  a  more  or  less  perfect  crystallisa- 
tion or  recrystallisation  of  the  constituents  of  the  rocks  affected,  leaving 
the  chemical  composition  practically  unchanged. 

It  is  only  generally  true,  however,  that  metamorphism  has  left  the 
chemical  composition  of  rocks  unchanged.  Not  infrequently,  silica  has 
been  introduced  in  abundance  from  batholiths,  so  as  to  permeate  the 
contiguous  rocks  and  to  fill  up  cracks  and  fissures.  The  rocks  of  the 
metamorphic  zone  are  thus  frequently  more  or  less  abundantly  traversed 
by  smaller  and  larger  veins  of  quartz,  which  in  places  may  extend  out- 
wards almost  to  the  very  margin  of  the  zone,  but  they  rarely  go  beyond 
it.  In  some  cases,  these  quartz-veins  are  accompanied  by  new  minerals, 
the  composition  of  which  shows  that  they  could  not  have  been  derived 
from  the  alteration  of  the  surrounding' rocks.  Among  the  most  interesting 
examples  are  the  tin-bearing  veins  which  are  associated  with  intrusive 
masses  of  granite  and  other  acid  eruptives,  and  the  apatite-veins  which 
are  more  particularly  connected  with  batholiths  of  gabbnx  In  the 
formation  of  the  cassiterite- veins,  various  volatile  fluorides,  boron- 
compounds,  etc.,  have  taken  part ;  for  the  tin-ore  is  usually  accompanied 
by  fluor-spar,  schorl,  etc.  According  to  Professor  Vogt,  the  contents 
of  such  veins  were  extracted  from  the  granite  before  the  plutonic  mass 
had  fully  congealed.  This  is  proved  by  the  fact  that  the  same  series 
of  elements  which  characterise  the  cassiterite-veins  occur  also  in  the 
pegmatite-veins  of  granite.  In  the  case  of  the  apatite- veins,  analogous 
phenomena  occur,  the  elements  they  contain  being  the  same  as  those 
met  with  in  gabbro.  Thus,  while  potassium  and  lithium  minerals  are 
characteristic  of  tin-veins,  magnesium  and  calcium-sodium  minerals 
are  notable  constituents  of  apatite-veins.  "  In  both  classes  of  veins," 


ALTERATION  AND  METAMORPHISM  219 

Vogt  remarks,  "we  find  a  characteristic  pneumatolytic  metamorphism 
of  the  country-rock.  Each  class  has  in  abundance  a  halogen  element, 
the  tin-veins  carrying  fluorine  (with  a  very  little  chlorine),  and  the 
apatite-veins  chlorine  (with  a  very  little  fluorine)."  He  concludes, 
therefore,  that  the  materials  of  the  apatite-veins  have  been  extracted 
from  the  gabbro  magma,  just  in  the  same  way  as  the  contents  of  the  tin- 
veins  have  been  obtained  from  granite.  In  the  former  case,  an  aqueous 
hydrochloric  solution  has  been  concerned  in  the  extraction  process,  while 
in  the  latter  case  this  process  has  been  based  chiefly  upon  a  reaction  in  the 
presence  of  water  of  hydrofluoric  acid  dissolved  in  the  granite  magma. 

Not  improbably,  many  other  veins,  rich  in  ores  of  various  kinds, 
which  occur  in  close  association  with  eruptive  rocks,  have  originated  in 
the  same  way  as  the  tin-veins  and  apatite-veins.  The  veins  referred  to 
are  usually  independent  of  the  character  of  the  rocks  they  traverse, 
while  a  more  or  less  clear  genetic  connection  can  be  established  between 
them  and  the  eruptive  masses.  Moreover,  the  rocks  in  which  they  occur 
are  always  metamorphosed  in  a  less  or  greater  degree  ;  they  have 
obviously  been  permeated  by  mineralising  agents,  or  subjected  to  a 
kind  of  solfataric  action.  (See  further  under  "  ORE-FORMATIONS.") 

The  following  conclusions  appear  to  be  well  established  as 
a  result  of  the  study  of  Thermal  or  Contact  Metamorphism  : — 

1.  Rocks  of  all  kinds  are  liable  to  become  metamorphosed 
at  their  contact  with  eruptives — the   nature  of  the  changes 
depending  partly  on  the  chemical  composition  of  the  invaded 
rocks,  and   partly  on    the   petrographical   character  and  the 
volume  of  the  intrusive  masses. 

2.  Metamorphism  has  usually  been  effected  without  any 
marked    alteration  of  the  chemical  composition  of  the  rocks 
attacked. 

3.  In   certain    cases,    however,   highly   heated    solutions, 
derived  from  plutonic  intrusions,  have   penetrated   and  per- 
meated  contiguous    and    surrounding    rocks,   and    thus,   by 
introducing  new  materials,  have  altered    more   or   less  con- 
siderably their  chemical  composition. 

4.  Crystallisation     has     been     superinduced     by    meta- 
morphism   in   derivative    rocks,  while  igneous  and  schistose 
rocks  have  in  like  manner  been  recrystallised. 

5.  The  production  of  new  minerals  is  a  common  accom- 
paniment of  thermal  metamorphism. 

6.  Now    and  again  the   rocks  near   their  contact  with  a 
batholith  may  be  rendered  schistose,  owing  to  the  develop- 
ment of  new  minerals  along  pre-existing  planes  of  division, 
whether  planes  of  bedding,  cleavage,  or  foliation. 


220  STRUCTURAL  AND  FIELD  GEOLOGY 

7.  The  petrographical  character  of  a  batholith  is  some- 
times considerably  affected  by  that  of  the  rocks  it  has  invaded. 
Apparently  this  is  due  to  the  latter  having  been  to  some 
extent  absorbed  and  assimilated  by  the  intrusive  mass. 

(£)  Regional  Metamorphism.  -  -  There  are  extensive 
regions  of  schistose  rocks  where  plutonic  masses  are  so 
sparingly  present  that  the  metamorphism  can  hardly  be 
assigned  to  their  action.  When  gneiss,  mica-schist,  etc.,  are 
found  occupying  hundreds  and  even  thousands  of  square 
miles,  it  is  impossible  to  believe  that  such  broad  areas  could 
have  been  affected  by  the  more  or  less  widely  separated 
batholiths,  sills,  and  dykes  by  which  they  are  often  traversed. 
Alteration  on  this  extensive  scale  is  known  as  Regional 
Metamorphism.  There  have  been  many  speculations  as  to 
its  cause  or  causes.  Some  geologists,  indeed,  are  inclined  to 
the  view  that  regional  metamorphism  is  only  contact  or 
thermal  metamorphism  "  writ  large,"  as  it  were.  They  hold 
it  probable  that,  although  intrusive  rocks  may  appear  at  the 
surface  only  here  and  there  throughout  an  extensive  area  of 
schistose  rocks,  nevertheless  such  an  area  may  be  underlaid 
at  no  great  depth  by  vast  plutonic  masses.  It  is  impossible 
to  deny  that  this  may  sometimes  or  even  often  be  the  case. 
There  can  be  no  doubt  that  batholiths  which  show  at  the 
surface  frequently  extend  laterally  for  long  distances  under- 
ground, and  this  is  one  reason  for  the  extreme  width 
sometimes  attained  by  the  aureole  of  metamorphic  rocks 
surrounding  a  plutonic  mass.  It  is  probable,  moreover,  that 
the  numerous  veins  and  dykes  which  often  crop  out  at  a 
great  distance  from  the  visible  margin  of  a  granite  mass  are 
not  directly  connected  with  it,  but  with  its  underground 
extensions.  Nevertheless,  when  throughout  an  extensive 
region  of  schistose  rocks  no  batholiths  appear,  even  in  the 
deepest  sections,  while  dykes  are  either  absent  or  very 
sparingly  present — we  are  not  justified  in  assuming  the 
existence  of  concealed  plutonic  masses  to  account  for  the 
general  metamorphism.  Cases  of  this  kind  call  for  a  different 
explanation. 

(a)  Plutonic  Metamorphism. — The  earliest  attempt  to  explain  the 
phenomena  in  question  was  made  by  Hutton — the  eminent  Scottish 
geologist — who  maintained  that  the  crystalline  schists  were  originally 


ALTERATION  AND  METAMORPHISM  221 

aqueous  sediments  which  had  been  gradually  deposited  upon  the  floor 
of  the  ocean.  When  a  great  thickness  of  strata  had  accumulated,  the 
loose  sediments  were  supposed  to  have  been  consolidated  by  the  pressure 
of  the  overlying  masses.  The  internal  heat  of  the  earth  next  began  to 
soften  the  compressed  strata,  and  even  eventually  to  melt  them.  The 
melted  portions  were  thought  to  be  now  represented  by  granite,  etc., 
while  the  strata  which  were  only  softened  by  the  "internal  fire,"  now 
formed  our  crystalline  schists.  The  view  held  by  Hutton  and  his  followers 
still  finds  many  supporters.  But  with  our  increased  knowledge  of  the 
geological  structure  of  the  earth's  crust,  and  of  the  chemical  and  physical 
conditions  which  have  played  their  part  in  modifying  rocks,  it  is  needless 
to  say  that  the  views  of  plutonic  metamorphism  now  maintained  differ 
very  considerably  from  those  first  enunciated  by  Hutton. 

The  changes  which  affect  the  crust  superficially,  as  we  have  seen,  are 
the  result  of  weathering,  and  are  brought  about  at  ordinary  temperatures 
and  under  atmospheric  pressure  only.  But  temperature  and  pressure 
gradually  augment  with  increasing  depth.  At  first  they  are  both 
moderate,  and  water  is  plentifully  present.  Hence  the  chemical  pro- 
cesses taking  place  in  this  upper  zone  might  be  expected  to  result  in 
the  formation  of  many  common  minerals,  especially  hydrates,  such  as 
hydrous-mica,  chlorite,  talc,  etc.,  together  with  magnetite,  quartz,  calcite, 
etc.  To  this  zone,  therefore,  should  belong  such  rocks  as  hydro-mica- 
schist,  phyllite,  chlorite-schist,  talc-schist,  serpentine,  quartzite,  etc.  At 
a  greater  depth  the  mineralogical  changes  must  become  more  marked — 
among  the  metamorphic  rocks  developed  in  this  second  or  middle 
zone,  would  be  mica-schist,  staurolite-schist  and  amphibole-schists, 
garnet-rock,  mica-gneiss,  hornblende-gneiss,  marble,  quartzite,  etc.  In 
the  deepest  zone  under  a  very  high  temperature  and  excessive  pressure 
the  metamorphism  ought  to  be  correspondingly  increased.  Here,  owing 
to  the  meagre  presence  of  water,  a  general  absence  of  hydrates  might 
be  expected — and  the  rocks  most  characteristic  of  this  zone  should  be 
gneisses  of  various  kinds  (biotite-,  augite-,  sillimanite-,  cordierite-gneiss), 
garnet-rocks,  marble,  quartzite,  etc.  In  short,  the  metamorphism  would 
gradually  increase  in  intensity  as  the  highly  heated  interior  was 
approached.  It  is  even  conceivable  that  at  the  greatest  depths  the 
metamorphosed  rocks  might  be  melted. 

Thus  the  theory  of  plutonic  metamorphism  does  not,  after  all,  differ 
essentially  from  that  of  contact  metamorphism,  for,  according  to  the 
former,  the  heated  interior  of  the  earth  seems  to  have  played  the 
same  role  as  a  batholith.  If  the  theory  in  question  were  generally 
applicable,  then  it  would  follow  that  all  rocks  which  have  formerly  been 
covered  by  a  great  thickness  of  overlying  masses,  and  thus  brought 
within  the  influence  of  a  high  subterranean  temperature,  ought  to  be 
more  or  less  metamorphosed  ;  while  strata  of  relatively  recent  date, 
which  never  could  have  been  thus  deeply  buried,  ought  to  be  free  from 
any  trace  of  metamorphism.  As  matter  of  fact,  however,  there  are 
wide  regions  occupied  by  great  successions  of  sedimentary  rocks — the 
basement  beds  of  which,  owing  to  folding  and  subsequent  denudation, 


222  STRUCTURAL  AND  FIELD  GEOLOGY 

are  now  exposed  ;  but  although  those  lower  beds  must  have  been  subject 
to  the  action  of  plutonic  heat,  they  yet  remain  unaltered.  On  the  other 
hand,  much  younger  formations,  which  have  not  been  concealed  under 
any  considerable  thickness  of  rock,  have  nevertheless  in  some  cases  been 
highly  metamorphosed. 

Certain  recent  observations  in  Finland,  by  Dr  J.  J.  Sederholm,  would 
seem  to  show,  however,  that  the  old  Huttonian  view,  as  subsequently 
modified,  may  have  greater  significance  than  many  geologists  have 
recognised.  Dr  Sederholm  sets  forth  certain  remarkable  evidence 
which  has  led  him  to  conclude  that  wholesale  "  refusion  or  resolution  " 
of  certain  pre-Cambrian  rocks  (consisting"  of  granitoid  gneisses  with 
subordinate  sedimentary  strata)  has  actually  taken  place.  According 
to  him  this  melting  process  must  have  been  effected  at  a  time  when 
these  rocks  were  buried  under  a  great  thickness  of  rock-masses,  removed 
since  by  denudation.  The  pre-Cambrian  strata  are  believed  to  have 
been  so  deeply  depressed  that  they  approached  the  highly  heated  interior 
or  "bottomless  magma  ocean"  of  the  earth.  Under  such  conditions. 
the  rocks  in  question  appear  to  have  been  largely  melted  or  resorbed 
by  the  magma,  and  thus  eventually  transformed  into  crystalline  granitoid 
masses.  Through  these  are  dispersed  isolated  fragments  (xenoliths)  of 
the  original  rocks  which  are  often  fused  to  such  an  extent  that  they  are 
almost  effaced. 

(b)  Hydrochemical  Metamorphism. — In  opposition  to  the  views  upheld 
by  the  supporters  of  plutonic  metamorphism,  Bischoff,  in  his  famous 
work  (Chemical  and  Physical  Geology],  maintained  that  high  temperature 
and  pressure  were  not  required  to  account  for  the  phenomena  of  the 
crystalline  schists.  He  showed  that  water  slowly  percolating  through 
the  rocks  would  act  as  a  reagent — breaking  up  minerals  and  inducing 
multitudinous  recombinations,  and  that  all  the  constituent  minerals  of 
schistose  rocks  could  be  produced  in  the  wet  way  at  ordinary  tempera- 
tures. His  conclusions  were  largely  based  on  the  study  of  pseudomorphs, 
which  he  had  no  difficulty  in  showing  frequently  occurred  in  rocks 
that  gave  no  evidence  of  having  been  subjected  to  heat.  One  mineral 
could  be  altered  into  another  either  by  the  loss  or  the  gain  of  an  ingredient, 
or  by  the  exchange  of  ingredients.  Or  there  might  be  a  total  change 
of  substance — the  new  mineral  containing  none  of  the  ingredients  of  its 
predecessor.  If  this  could  be  the  case  with  crystallised  minerals, 
similar  changes  must  affect  sedimentary  rocks — out  of  clay,  for  example, 
all  the  minerals  of  gneiss  might  be  developed  by  chemical  reactions. 
The  hydro-chemical  theory  is  thus  plausible  enough,  and  explains  many 
of  the  alterations  which  all  rocks  have  undergone.  Bischoff's  work  was 
of  essential  service,  and  must  still  be  studied  by  geologists  who  are 
interested  in  the  remarkable  transformations  which  are  brought  about 
by  the  action  of  meteoric  water  making  its  way  down  from  the  surface. 
The  theory  fails,  however,  to  account  for  regional  metamorphism.  If 
it  were  well  founded,  then  all  the  oldest  sedimentary  formations  should 
long  ago  have  been  metamorphosed,  while  the  younger  systems  should 
never  show  any  trace  of  such  change.  Yet  we  find  that  in  many  places 


ALTERATION  AND  METAMORPHISM  223 

the  very  oldest  fossiliferous  strata  (Cambrian),  although  they  must  have 
been  subject  to  the  action  of  percolating  water  for  untold  millions  of 
years,  are  nevertheless  quite  unchanged.  On  the  other  hand,  strata 
belonging  to  relatively  recent  times  (Tertiary)  have  in  some  places  been 
rendered  crystalline  and  schistose.  Even  if  these  contradictory  facts 
could  be  reconciled  or  explained  away,  we  should  still  be  unable  to 
explain  the  appearance  presented  by  the  schists  themselves.  These,  as 
we  have  seen,  are  arranged  in  layers  or  beds  of  very  different  chemical 
and  mineralogical  constitution  —  mica-schist,  for  example,  is  found 
alternating  with,  but  sharply  marked  off  from,  beds  and  layers  of  such 
rocks  as  hornblende-schist,  talc-schist,  gneiss,  quartzite,  serpentine, 
crystalline  limestone,  etc.  Had  the  metamorphism  of  these  rocks  been 
caused  by  circulating  water  introducing  and  abstracting  ingredients,  as 
in  the  formation  of  pseudomorphs,  there  could  have  been  no  such 
arrangement  of  the  schists  as  that  referred  to.  The  changes  effected  by 
percolating  water  would  have  been  independent  of  bedding-planes,  and 
would  have  been  most  in  evidence  along  the  more  or  less  vertical  joints 
and  fissures  by  which  the  rocks  are  traversed. 

(c.)  Dynamo-metamorphism. — New  light  was  thrown  upon  the  origin 
of  regional  metamorphism,  when  it  became  recognised  that  the  altered 
rocks  were  usually  somewhat  highly  folded,  and  that  the  intensity  of  the 
metamorphism  was  in  direct  proportion  to  the  degree  of  crustal  deforma- 
tion— crystalline  texture  and  schistose  structure  becoming  more  and 
more  pronounced  as  the  centres  or  axes  of  greatest  disturbance  were 
approached.  It  was  observed  that  in  the  areas  of  greatest  disturbance 
highly  crystalline  and  puckered  schistose  rocks  predominated,  and  that 
as  one  passed  outwards  from  such  areas,  rocks  of  that  type  gradually 
gave  place  to  others  in  which  crystalline  texture  and  foliated  structure 
became  less  and  less  prominent,  and  at  last  died  away  as  flexing,  folding, 
and  rock-displacements  continued  to  diminish  in  importance. 

The  effects  produced  by  this  dynamo-metamorphism  resemble  in 
some  respects  those  brought  about  by  thermal  metamorphism.  In  both 
cases  alike,  the  changes  have  as  a  rule  left  unaltered  the  composition  of 
the  rocks  attacked.  Clastic  rocks,  owing  to  recombinations  of  their 
ingredients,  have  been  rendered  crystalline,  while  igneous  and  old 
schistose  rocks  have  in  like  manner  been  recrystallised.  In  other 
respects,  however,  there  are  notable  differences  to  be  observed  between 
the  two  kinds  of  metamorphism.  In  regional  metamorphism,  for 
example,  we  have  no  evidence  of  actual  fusion,  such  as  occurs  now  and 
again  in  the  case  of  thermal  or  contact  metamorphism.  On  the  other 
hand,  in  contact  metamorphism  there  is  little  to  show  that  the  altered 
rocks  have  been  concurrently  subjected  to  much  lateral  pressure,  whereas 
the  rocks  throughout  an  area  of  dynamo-metamorphism  give  proof  of 
having  experienced  the  most  intense  compression.  Again,  in  the  case 
of  contact  metamorphism,  foliation  when  present  always  coincides  in 
direction  with  pre-existing  planes  of  division,  while  in  that  of  regional 
metamorphism  such  coincidence  is  more  or  less  accidental,  foliation 
having  been  developed  usually  along  planes  of  compression.  In  steeply 


224  STRUCTURAL  AND  FIELD  GEOLOGY 

folded  rocks,  therefore,  foliation  sometimes  coincides  with  original  bedding- 
planes,  or  it  may  cross  these  at  any  angle.  Owing  to  metamorphism, 
however,  the  original  rock-structures  a.re  often  wholly  obliterated, 
and  it  is  then  impossible  to  say  what  influence  these  may  have  had 
in  determining  the  direction  of  foliation.  Along  great  thrust-planes  the 
immediately  adjacent  rocks  are  often  rendered  crystalline  and  schistose, 
and  in  such  cases  the  foliation  coincides  in  direction  with  the  plane  of 
rock  displacement. 

Slaty  Cleavage. — In  a  preceding  chapter  (p.  141)  the 
phenomena  of  rock-folding  were  discussed,  and  it  was  there 
pointed  out  that  the  constituent  ingredients  of  a  folded  rock 
were  often  more  or  less  deformed  or  distorted.  Deformation 
of  the  kind  referred  to  is  often  conspicuously  developed  in 
areas  of  dynamo-metamorphism,  more  especially  along  their 
outer  margin.  In  this  peripheral  zone  the  rocks  may  be 
arranged  in  more  or  less  steeply  inclined  positions,  but  they 
are  neither  crystalline  nor  foliated.  Nevertheless  they  usually 
afford  evidence  of  having  been  compressed.  This  is  shown 
by  the  superinduced  structure  known  as  Slaty  Cleavage,  a 
structure  which  renders  a  rock  capable  of  being  cleaved  or 
split  into  slabs,  plates,  or  laminae  in  a  direction  independent 
of  the  planes  of  bedding.  When  such  a  rock  is  examined 
under  the  microscope,  the  particles  of  which  it  is  composed 
are  seen  to  be  flattened  out  in  one  and  the  same  direction 
— an  arrangement  which  obviously  accounts  for  the  fissile 
character  of  clay-slate.  A  rock  of  this  kind,  therefore,  cleaves 
or  divides  most  readily  along  planes  of  compression,  and  not, 
as  in  the  case  of  shale,  along  planes  of  deposition.  That 
slaty  cleavage  is  one  of  the  concomitant  results  of  crustal 
deformation  is  shown  by  the  fact  that  the  planes  of  cleavage 
are  always  parallel  to  the  axes  of  anticlinal  and  synclinal 
folds.  When  the  structure  is  well  developed,  not  only  does 
the  original  lamination  disappear,  but  even  the  planes  of 
bedding  may  be  rendered  obscure  or  altogether  obliterated. 
Cleavage  may  intersect  the  bedding-planes  at  any  angle,  or 
may  now  and  again  coincide  with  these  where  the  limb  of  a 
fold  is  inclined  in  the  same  direction  and  at  the  same  angle  as 
the  planes  of  compression  (see  Plate  XLIX.).  Slaty  cleavage 
is  best  developed  in  fine-grained,  homogeneous  clay-rocks, 
which  are  sometimes  so  fissile  that  they  divide  with  ease  into 
very  thin  smooth  plates.  It  is  not  confined,  however,  to 


[To  face  page  224. 


ALTERATION  AND  METAMORPHISM  225 

argillaceous  strata,  but  may  affect  rocks  of  the  most  diverse 
character,  as  greywacke,  conglomerate,  and  crystalline  erup- 
tives  ;  but  in  such  rocks  it  is  never  well  developed,  the  planes 
of  cleavage  being  usually  imperfect  and  more  or  less  irregular 
and  discontinuous. 

Although  the  clastic  character  of  ordinary  clay-slate  is 
sufficiently  obvious,  the  rock  is  nevertheless  not  quite  devoid 
of  all  crystalline  structure.  Now  and  again  the  surfaces  of 
the  cleavage-planes  show  scales  of  mica  and  needles  of  rutile, 
and  such  indications  of  incipient  metamorphism  gradually 
increase  as  the  centre  or  axial  zone  of  a  much  disturbed 
region  is  approached,  with  the  result  that  clay-slate  merges 
into  phyllite.  Followed  in  the  same  direction,  phyllite  in  its 
turn  passes  into  mica-schist,  while  the  foliation  of  the  latter 
may  become  more  and  more  puckered  and  crumpled  as  the 
contortion  of  the  rocks  increases.  Finally,  the  mica-schist 
may  merge  into  a  gneiss. 

The  changes  involved  in  the  passage  of  a  clay-rock  through  the 
several  stages  of  slate,  phyllite,  schist,  and  gneiss  are  obviously  partly 
mechanical,  partly  chemical.  No  doubt  the  rocks  undergoing  deforma- 
tion would  be  more  or  less  deeply  buried,  and  subject  therefore  to  the 
pressure  of  overlying  masses  and  possibly  also  to  the  action  of  the 
internal  heat  of  the  earth.  However  that  might  be,  it  is  obvious  that 
while  the  process  of  flexing  and  folding  was  going  on,  heat  would 
necessarily  be  evolved  and  continue  to  augment  as  compression  in* 
creased.  The  ingredients  of  the  rocks  would  be  mechanically  crushed 
and  flattened,  while  at  the  same  time  chemical  action  would  be  stimu- 
lated, and  in  the  presence  of  water  recombinations  of  the  rock-materials 
would  be  effected — the  minerals  thus  formed  being  arranged  with  their 
longer  axes  parallel  to  the  planes  of  compression.  Clay-rocks,  com- 
posed as  they  are  of  fine-grained  and  relatively  soft  ingredients,  would 
naturally  offer  least  resistance  to  compression — cleavage-structure  would, 
in  their  case,  be  readily  superinduced.  On  the  other  hand,  coarse- 
grained rocks,  whether  clastic  or  crystalline,  would  not  be  so  readily 
affected.  Their  constituents  being  individually  larger  and  usually  more 
resistant  than  those  of  an  argillaceous  rock,  a  greater  degree  of  pressure 
would  be  required  to  crush  and  flatten  them.  Hence  it  is  that  coarse- 
grained beds  interstratified  with  clay-slates  often  show  little  or  no  trace 
of  change  beyond  mere  induration.  When  such  coarse-grained  rocks, 
however,  are  followed  towards  the  zone  or  centre  of  greatest  disturbance, 
they  all  eventually  yield  and  become  cataclastic.  The  rounded  stones  of 
a  conglomerate  and  the  angular  fragments  of  a  breccia,  for  example,  are 
crushed,  flattened,  and  elongated  until  they  appear  as  mere  lenticular 
streaks  or  cease  to  be  recognisable,  stones  and  matrix  together  being 

P 


226  STRUCTURAL  AND  FIELD  GEOLOGY 

converted  into  a  mylonite  or  into  a  crystalline  schistose  aggregate. 
Granitoid  crystalline  rocks  are  in  like  manner  crushed  down,  recrystal- 
lised,  and  foliated.  Not  infrequently,  in  such  crushed  eruptives, 
lenticular  cores  (or  phacoids,  as  they  are  termed)  of  the  original  rock  can 
still  be  observed,  around  which  the  finely  pulverised  and  recrystallised 
materials  are  arranged  much  in  the  same  way  as  the  smaller  crystalline 
ingredients  of  a  lava  have  grouped  themselves  about  a  phenocryst.  All 
the  phenomena,  in  short,  conspire  to  show  that  the  metamorphosed 
rocks  in  question  have  been  so  compressed  and  crushed  that  they  have 
been  compelled  to  flow. 

Just  as  in  thermal  or  contact  metamorphism  the  rocks  become  in- 
creasingly affected  as  a  plutonic  mass  is  approached,  so  also  in  regional 
metamorphism  we  encounter  gradually  augmenting  rock-changes  while 
we  proceed  from  the  peripheral  areas  of  comparatively  unaltered  rocks 
to  the  entirely  reconstituted  masses  of  the  interior  region.  Advancing 
towards  the  latter  region  we  first  encounter,  it  may  be,  slates,  phyllites, 
hydrous  mica-schists,  chlorite-schists,  serpentine,  diabase-schist,  green 
schists,  conglomerate-schists,  and  other  rocks  similarly  indicative  of  less 
intense  metamorphic  action.  Next  we  enter  a  region,  the  most 
characteristic  rocks  of  which  are  andalusite-,  kyanite-,  and  staurolite- 
schists,  mica-  (muscovite,  biotite)  schists,  amphibole-schists  and 
amphibole-rock,  granulite,  gneisses  (usually  fine-grained),  etc.  Reach- 
ing the  inner  zone,  we  are  confronted  with  coarse  biotite-schist,  fre- 
quently containing  garnets,  granulite,  leclogite,  biotite-garnet-gneisses 
(often  coarse-grained),  hornblende-gneiss,  amphibolite,  etc.  Quartzite, 
crystalline  limestone,  and  calc-mica-schist  may  be  present  on  any 
horizon. 

Here,  then,  we  have  much  the  same  succession  of  changes  as  are 
supposed  to  have  occurred  in  the  case  of  plutonic  metamorphism.  And 
upholders  of  the  latter  theory  would  probably  claim  such  a  succession  as 
favouring  their  own  view.  The  present  folded  and  crumpled  aspect  of 
the  schists,  they  might  say,  were  the  result  of  subsequent  crustal 
deformation. 

The  theory  of  dynamo-metamorphism  explains  so  many  striking 
phenomena  which  are  hard  to  account  for  by  the  plutonic  theory  that  it 
is  accepted  by  many  geologists  as  giving  a  reasonable  interpretation  of 
regional  metamorphism  as  a  whole.  Nevertheless  there  are  difficulties 
in  the  way  of  accepting  it  as  generally  applicable.  For  example,  in 
many  places  the  highly  convoluted  strata  of  certain  mountains  of  uplift 
show  no  evidence  of  true  metamorphism — the  petrographical  character 
of  clay-slates,  greywackes,  sandstones,  and  limestones  has  remained 
unaffected  during  the  process  of  compressing  and  folding.  Again,  there 
are  regions  where  highly  crystalline  schists  occupy  undisturbed  positions 
— that  is  to  say,  they  are  not  plicated  or  folded.  Once  more,  it  has  been 
shown  that  the  process  of  metamorphism  has  in  some  cases  preceded 
that  of  rock-folding. 

The  probabilities  are  that  metamorphism  is  the  result  sometimes  of 
contact  with  batholiths  or  even  with  the  heated  interior  of  the  earth,  and 


ALTERATION  AND  METAMORPHISM  227 

sometimes  to  the  strains  and  stresses  due  to  crustal  movements. 
Occasionally  thermal  and  dynamo-metamorphism  may  have  acted  to- 
gether, and  in  such  cases  it  may  be  impossible  to  say  which  of  the  two  pro- 
cesses has  played  the  dominant  role.  In  both  pressure  is  recognised  as 
an  important  factor,  and  the  presence  of  water,  either  in  the  liquid  or 
the  gaseous  form,  is  another  essential  condition — water  itself  acting  as  a 
mineralising  agent  and  carrying  with  it  various  other  chemical  agents  of 
change.  But  the  phenomena  of  slaty  cleavage,  and  the  cataclastic  struc- 
tures so  frequently  met  with  amongst  crystalline  schists  are  clearly  the 
result  of  compression  and  crushing,  and  can  only  be  explained  by  the 
theory  of  dynamo-metamorphism.  Even  schistose  structure  on  the  large 
scale  can  hardly  be  accounted  for  without  pressure.  The  metamorphism 
of  rocks,  however,  is  still  far  from  being  satisfactorily  explained. 
Geologists  are  much  divided  in  opinion,  and  many  observations  and 
much  research,  as  well  chemical  and  physical  as  geological,  will  be 
required,  before  an  adequate  conception  of  the  subject  can  be  attained. 

Archaean  Rocks. — Under  this  head  are  included  a  remarkable  group 
of  coarsely  crystalline  gneissose  rocks,  the  origin  of  which  has  been  a 
fruitful  subject  of  discussion.  The  rocks  in  question,  although  termed 
gneiss,  are  not  truly  schistose  rocks.  They  show  a  banded  structure, 
indeed,  but  this  cannot  be  confounded  with  foliation,  but  is  suggestive 
rather  of  a  kind  of  fluxion-structure.  The  bands  in  question  somewhat 
resemble  those  streaky  layers  and  veins  so  commonly  present  in  certain 
massive  eruptive  rocks.  The  constituent  minerals  of  the  layers  referred 
to  seem  to  have  segregated  either  while  the  igneous  rock  was  in  motion 
or  after  it  had  ceased  to  move.  The  gneissose  rocks,  moreover,  ever 
and  anon  lose  their  banded  structure,  and  merge  into  massive  rocks, 
which  cannot  be  distinguished  from  granitoid  eruptives.  Not  only  so, 
but  they  frequently  behave  as  intrusive  rocks,  one  gneiss  cutting  across 
another.  These  and  other  appearances  lead  to  the  belief  that  the 
Archaean  granite-gneisses  are  of  igneous  origin.  They  underlie  the 
oldest  stratified  rocks  of  the  globe,  wherever  the  base  of  these  is  exposed, 
and  hence  are  thought  by  some  geologists  to  represent  the  original  crust 
formed  upon  the  surface  of  the  globe.  They  vary  much  in  composition — 
from  highly  acid  to  highly  basic.  In  some  places  they  appear  to 
alternate  with  truly  schistose  rocks  and  crystalline  limestones,  as  if 
all  belonged  to  one  and  the  same  series.  This  appearance,  however,  is 
perhaps  deceptive,  and  due  to  the  intrusive  character  of  the  gneisses.  It 
may  be  added  that  almost  everywhere  the  Archaean  rocks  yield  evidence 
of  having  been  subjected  to  powerful  deformation — they  have  frequently 
been  crushed,  pulverised,  recrystallised,  and  foliated. 


CHAPTER   XVI 

ORE-FORMATIONS 

Syngenetic  Ore-Formations — Native  Metals  and  Ores  in  Igneous  Rocks  ; 
Ores  in  Bedded  Rocks  (Chemical  Precipitates,  Clastic  Ores,  Ores 
in  Schists).  Epigenetic  Ore-Formations — Fissure  Veins  or  Lodes ; 
Nature  of  Fissures  ;  Width  and  Extent  of  Lodes  ;  Simple  and 
Complex  Lodes  ;  Transverse  and  Coincident  Lodes  ;  Systems  of 
Lodes  ;  Branching  and  Intersection  of  Lodes  ;  Heaving  of  Lodes  ; 
Contents  of  Fissure  Veins  ;  Structure  of  Fissure  Veins  ;  Outcrop  of 
Lodes ;  Gossans  ;  Association  of  Ores  in  Lodes ;  Succession  of 
Minerals  in  Lodes  ;  Walls  of  Lodes  ;  Stockvvorks. 

ORES  are  metalliferous  minerals  or  mixtures  of  such  minerals, 
in  which  the  proportion  of  metal  is  often  sufficiently  large  to 
admit  of  its  being  profitably  extracted.  The  term  "  metal  "  is 
here  used  in  a  conventional  (not  in  a  chemical)  sense,  and  does 
not,  therefore,  apply  to  the  metals  of  the  alkalies  and  alkaline 
earths,  but  only  to  the  "  heavy "  metals  of  commerce,  viz. : 
gold,  silver,  platinum,  copper,  tin,  lead,  zinc,  iron,  manganese, 
nickel,  cobalt,  chromium,  mercury,  antimony,  bismuth,  etc. 

Classification. — As  one  kind  of  ore-formation  frequently 
passes  into  another,  while  considerable  doubts  obtain  as  to 
the  genesis  of  many  ores,  it  is  hardly  possible  to  devise  a 
scheme  of  classification  to  which  exception  cannot  be  taken. 
For  purposes  of  description,  however,  ore-formations  may  be 
grouped  under  these  two  main  divisions: — i.  Syngenetic  or 
Contemporaneous,  and  2.  Epigenetic  or  Subsequent. 

I.— SYNGENETIC   ORE-FORMATIONS 

These  are  formations  of  the  same  age,  broadly  speaking, 
as  the  rocks  in  which  they  occur  or  with  which  they  are 
immediately  associated.  Some  of  them  appear  in  igneous 


ORE-FORMATIONS  229 

rocks,  while  others  are   associated   with   derivative,   and   yet 
others  with  schistose,  rocks. 

i.  ORES  OCCURRING  IN  IGNEOUS  ROCKS 
Ores  of  this  class  are  original  or  primary  constituents, 
appearing  sometimes  as  isolated  grains  or  crystals,  dissemin- 
ated through  the  body  of  a  rock ;  at  other  times,  as  larger  or 
smaller  aggregates  or  masses  which  have  obviously  separated 
out  from  a  molten  magma.  Not  only  ores  but  native  metals 
occur  under  these  conditions,  more  especially  in  plutonic  basic 
igneous  rocks.  Acid  plutonic  rocks,  on  the  other  hand,  arc 
seldom  rich  in  such  constituents. 

(i)  Native  Metals. — Iron  is  irregularly  disseminated 
through  the  basalt  of  Ovifak  (Disco  Island,  West  Greenland) 
in  the  form  of  scales,  grains,  nodules,  and  larger  lumps  and 
masses.  Nickel-iron,  in  small  grains,  is  met  with  in  peridotite 
and  serpentine  in  South  Island  (New  Zealand).  Platinum 
also  occurs  in  similar  small  grains  in  peridotites,  olivinc- 
gabbros,  and  certain  syenitic  rocks  in  the  Ural  Mountains, 
and  in  peridotite  in  British  Columbia.  Gold,  silver,  copper, 
etc.,  have  likewise  been  detected,  generally  as  minute  inclu- 
sions, in  the  constituent  minerals  of  various  igneous  rocks — 
never  in  sufficient  quantity,  however,  to  invite  mining 
operations. 

(2}  Ores. — These  are  oxides  and  sulphides — the  former 
being  represented  chiefly  by  magnetite,  ilmenite,  and 
chromite;  and  the  latter  by  pyrite,  pyrrhotite,  and  chalco- 
pyrite — each  of  which  may  contain  variable  percentages  of 
nickel  and  cobalt. 

Oxides.— Magnetite,  often  titaniferous,  is  one  of  the  commonest  and 
most  widely  diffused  constituents  of  igneous  rocks.  Now  and  again  it 
forms  massive  aggregates  in  plutonic  basic  eruptives,  as  in  certain 
gabbros,  and  gabbro-diorites  in  Sweden,  Finland,  Norway,  and  North 
America.  In  these  rocks  the  mineral  occurs  disseminated  in  the  usual 
way,  along  with  other  accessory  constituents,  but  is  so  abundant  as  some- 
times to  constitute  a  large  percentage  of  the  rock.  Here  and  there, 
indeed,  it  becomes  concentrated  so  as  to  form  enormous  aggregates.  In 
some  cases  these  aggregates  are  sharply  marked  off  from  the  igneous 
rock  in  which  they  lie  ;  in  other  cases,  the  disseminated  ore  gradually 
becomes  more  and  more  abundant  at  the  expense,  so  to  speak,  of  the 
other  constituents  of  the  gabbro,  so  that  there  seems  to  be,  as  it  were,  a 
passage  from  the  latter  into  the  ore-aggregate.  Such  aggregates  arc 


230 


STRUCTURAL  AND  FIELD  GEOLOGY 


usually  rich  in  ferromagnesian  minerals  (hornblende,  rhombic  pyroxene, 
and  olivine),  which  are  not  infrequently  accompanied  by  biotite,  apatite, 
green  spinel,  and  various  sulphides  (pyrite,  pyrrhotite,  and  chalcopyrite). 


FIG.  80. — SECTION  AT  BLAAFJELD.    (Vogt.) 
L,  Labradorite-rock ;  N,  Norite-pegmatite ;  I,  Ilmenite.    Length  of  Section  600  metres.* 

Ilmenite  (titaniferous  iron-ore)  occurs  in  some  Norwegian  gabbros  under 
similar  circumstances  (Fig.  80),  the  ore-aggregates   either  forming  an 

abrupt  junction  with  the 
mother  rock  or  graduating 
into  it  in  the  same  way 
as  titaniferous  magnetite. 
Chromite  is  a  frequent  con- 
stituent of  peridotites,  and 
now  and  again  so  largely 
abounds  that  the  rock  con- 
taining it  is  mined.  At 
Hestmando,  in  Norway,  for 
example,  the  rock  exploited 
is  composed  essentially  of 
olivine,  enstatite,  picotite, 
and  chromite.  Tin-ore  (cas- 
siterite)  likewise  occurs  as  a 
primary  constituent  of  many 
granites,  but  only  in  scat- 
tered grains  and  thin  veins. 


*  The  Labradorite-rock 
(gabbro)  contains  some  2 
per  cent,  ilmenite,  4  per 
cent,  ferromagnesian  mine- 
rals, and  94  per  cent,  fel- 
spar ;  the  Norite-pegmatite 
yields  40  per  cent,  ilmenite, 
35  per  cent,  ferromagnesian 
minerals,  and  25  per  cent, 
felspar.  Where  the  Norite- 
pegmatite  graduates  into 

the  Labradorite-rock,  the  percentages  are  as  follows  :— 6  to  18,  ilmenite  ; 

8  to  1 6,  ferromagnesian  minerals  ;  and  56  to  66,  felspar. 


FIG.  81. — SKETCH-PLAN  OF  MEINKJAR, 
NORWAY.     (After  Prof.  Vogt.) 

s,  gneissose  rocks ;  7i,  hornblende-schist ;  n,  norite 
with  inclusions  of  gneiss  (xenoliths);  o,  ore; 
A,  B,  sections  across  the  area,  from  east  to  west. 


ORE-FORMATIONS  231 

It  has  never  been  mined  in  these  rocks,  but  this  is  probably  one  of  the 
sources  of  the  tin-ore  of  "  placers." 

Sulphides. — Pyrite  and  pyrrhotite  appear  now  and  again  as  in- 
gredients of  certain  igneous  rocks,  and  chalcopyrite  has  also  been 
recorded  as  occasionally  occurring  under  similar  conditions.  While,  in 
some  cases,  such  metallic  sulphides  may  be  of  secondary  origin,  there 
seems  no  reason  to  doubt  that  they  are  frequently  primary  constituents 
of  the  rocks  in  which  they  appear.  It  is  highly  probable,  therefore,  that 
the  massive  aggregates  of  sulphide  ores  met  with  in  certain  plutonic 
rocks  are  examples  of  magmatic  segregation,  and  as  truly  syngenetic  as 
the  magnetic  and  titaniferous  iron -ores  referred  to  above.  In  some 
of  the  Norwegian  gabbros,  pyrrhotite,  pyrite,  and  chalcopyrite,  each 
containing  a  variable  percentage  of  nickel  and  cobalt,  are  disseminated 
in  small  grains  through  the  rock,  but  now  and  again  they  have  segregated 
to  form  large  masses  of  irregular  form,  which  are  grouped  chiefly  along 
the  line  of  junction  between  the  gabbros  and  the  adjacent  rocks  (see  Fig. 
81).  Similar  examples  of  the  magmatic  segregation  of  nickeliferous 
sulphide-ores  are  met  with  in  Sweden,  Piedmont,  and  North  America. 
It  is  believed  by  some  authorities  that  the  auriferous  pyrite  of  Rossland, 
British  Columbia,  and  the  high-grade  copper-ores  occurring  in  the 
peridotites  and  serpentines  of  northern  Italy  have  originated  in  the  same 
way. 

2.  ORES  OCCURRING  IN  BEDDED  ROCKS 

Under  this  head  are  included  precipitates  from  aqueous 
solution,  certain  alluvial  deposits,  and  ores  interstratified  with 
crystalline  schists. 

(i)  Precipitates  from  Aqueous  Solution. — The  most  im- 
portant ores  of  this  origin  are  iron-  and  manganese-ores.  The 
iron-ores  in  question  are  well  represented  by  the  formations 
which  ar.e  taking  place  now  in  marshy  land  and  lakes.  These 
consist  essentially  of  hydrated  ferric  oxide,  but  usually  contain 
many  impurities.  Sometimes  they  form  continuous  beds ;  in 
other  places  they  occur  as  nodular  concretions  of  some  size, 
or  as  aggregates  of  oolitic  and  pisolitic  spherules.  They  are 
the  products  of  the  alteration  of  ferriferous  minerals  and  rocks, 
and  owe  their  origin  mainly  to  the  action  of  water  containing 
organic  acids,  which  act  as  powerful  solvents  of  iron-salts. 
Rocks  exposed  to  the  action  of  such  acidulated  water  are 
bleached  white  by  the  removal  of  their  iron,  which  is  carried 
away  in  solution  as  a  bicarbonate.  From  this  solution,  the 
iron  tends  to  be  precipitated  as  ferric  hydrate,  unless  much 
decomposing  organic  matter  be  present ;  when  such  is  the 


232  STRUCTURAL  AND  FIELD  GEOLOGY      4 

case  oxidation  is  prevented,  and  the  iron  is  then  thrown  down 
as  a  carbonate.  The  pisolitic  limonite  forming  in  the  shallow 
waters  of  many  existing  lakes,  and  the  earthy  bog  iron-ore 
so  frequently  present  in  swampy  land,  are  good  examples  of 
this  class  of  ore-formations.  Bedded  iron-ores  (both  oxides 
and  carbonates)  are  met  with  in  many  geological  systems — 
ranging  from  post-Tertiary  to  Palaeozoic  horizons.  While 
many  of  these  are  of  freshwater  or  brackish-water  origin, 
others  are  marine.  As  examples  may  be  cited  the  iron-ore 
of  Rio  Tinto,  in  the  province  of  Huelva,  Spain — a  deposit  of 
Recent  age ;  the  Mesozoic  limonites  and  earthy  carbonates 
of  the  Lias,  the  Great  Oolite,  the  Wealden,  and  the  Lower 
Greensand,  in  England ;  and  the  clay-ironstones  which  are  so 
abundantly  developed  in  the  Carboniferous  system  of  this 
and  other  countries  (Fig.  82).  Most  of  the  ironstones  last 


FIG.  82.— SEAMS  AND  NODULES  OF  CLAY-IRONSTONE  IN 
CARBONIFEROUS  SHALES. 

referred  to  appear  to  have  been  formed  by  direct  precipitation 
in  lakes  and  lagoons.  In  the  case  of  the  nodular  concretions 
met  with  in  the  same  series  of  strata,  we  have  examples  of 
the  subsequent  concentration  or  aggregation  of  ferruginous 
matter,  originally  diffused  through  the  beds  in  which  such 
nodules  occur. 

Manganese  ores  (pyrolusite,  psilomelane,  wad)  are  not  so 
abundantly  met  with  as  iron-ores.  They  occur,  however,  under 
similar  conditions  amongst  sedimentary  rocks  of  all  ages,  some- 
times as  concretionary  nodules,  at  other  times  in  layers  and 
beds,  which  are  not  infrequently  pisolitic. 

(2)  Clastic  Ore- Formations. — These  are  alluvial  deposits, 
derived  from  the  disintegration  of  metalliferous  rocks  and 
ore-bodies  of  various  origin,  and  are  known  to  mining  men  as 
Placers  (Fig.  83).  The  metals  obtained  from  such  deposits 
are  chiefly  gold,  platinum,  and  tin.  The  beds  vary  much  in 


ORE-FORMATIONS  233 

character,  consisting,  in  some  places,  of  coarse  gravel  and 
shingle,  or  of  finer  gravel,  grit,  and  sand.  Most  placers  are 
of  Recent  and  Pleistocene  age,  and  are  usually  more  or  less 
unconsolidated.  Many,  however,  occur  in  the  Tertiary 
system,  while  a  few  date  back  to  Mesozoic,  and  some  even  to 
Palaeozoic  times.  These  older  deposits  are,  as  a  rule,  con- 
solidated, forming  coarse  grits  and  conglomerates.  The 
metals  and  ores  of  highly  porous  placers  are  usually  con- 
centrated in  the  bottom  layers.  In  the  case  of  finer  grained 
alluvia,  however,  they  may  be  sparingly  scattered  through  the 
whole  thickness  of  the  deposits.  Should  the  bed-rock  under- 
lying a  placer  be  more  or  less  fissured  and  shattered,  the 
metal  or  ore  not  infrequently  finds  its  way  down  for  a  few 
inches  into  cracks  and  crevices.  While  the  gold  occurring  in 
quartz- veins,  etc.,  is  often  intimately  associated  with  metallic 


/////////  AC////////////  //////////// 


7///////II]  Illlllllllllllinilini- 


FIG.  83. — SECTION  OF  AURIFEROUS  LEAD  (OR  PLACER)  ON  THE  LOWER 
MURRAY,  NEAR  COROWA.    (After  E.  F.  Pittman.) 

sulphides,  such  as  iron-pyrite,  the  gold  met  with  in  placers  is 
usually  in  the  free  state.  During  the  processes  of  disintegra- 
tion and  denudation,  the  sulphides  containing  the  gold  are 
gradually  dissolved,  and  the  process  of  solution  is  carried  on 
in  the  placer  itself,  so  that  sooner  or  later  the  gold  becomes 
freed  from  its  baser  associates.  The  crystalline  surfaces 
occasionally  presented  by  placer-gold,  and  the  usually  smooth 
and  unscratched  appearance  of  the  nuggets,  are  suggestive  of 
chemical  deposition.  Many  mining  men,  indeed,  believe  that 
nuggets  grow  by  slow  accretion. 

Placers  being  of  fluviatile  origin,  it  will  be  readily  under- 
stood that  such  formations  can  seldom  be  of  great  geological 
antiquity.  Terrestrial  accumulations  are  only  exceptionally 
preserved — the  Mesozoic  and  Palaeozoic  systems  consist  for 
the  most  part  of  marine  formations.  The  further  back  we 
trace  the  geological  record,  therefore,  the  scantier  become  all 


234  STRUCTURAL  AND  FIELD  GEOLOGY 

traces  of  old  land-surfaces.  Lacustrine  and  fluviatile  deposits 
of  Tertiary  age  have  now  and  again  been  preserved,  under 
lava,  as  in  California  and  Victoria  (Australia).  The  auri- 
ferous gravels  of  these  regions  are  believed  to  be  river- 
gravels  belonging  to  the  Pliocene.  They  are  often  more  or 
less  hardened  by  infiltration  of  silica,  ferruginous  matter,  etc., 
and  constitute  the  "  deep  leads  "  of  the  miners.  The  shallow 
placers  of  the  same  regions  are  of  recent  age — derived  in 
considerable  measure  from  the  denudation  of  the  older  series. 
The  alluvial  deposits  of  the  Ural  Mountains,  which  yield  both 
gold  and  platinum,  the  "stream-tin"  (cassiterite)  accumula- 
tions of  Cornwall,  which  are  now  practically  exhausted,  and 
the  alluvial  tin-fields  of  Malaysia,  from  which  three-fourths  of 
the  world's  output  at  present  come — are  all  examples  of  the 
same  class  of  ore-formations.  Placers  of  older  date  than  the 
Tertiary  are  of  rare  occurrence,  only  a  few  gold-bearing  con- 
glomerates having  been  met  with  in  Mesozoic  and  Palaeozoic 
systems,  and  these  are  seldom  rich  enough  to  be  worked. 

(3)  Ores  occurring  in  Schistose  Rocks. — Ores  of  iron 
and  manganese  are  the  most  frequently  occurring  formations 
met  with  as  beds  interstratified  with  schistose  rocks.  It  is 
sometimes  difficult  to  distinguish  such  syngenetic  formations 
from  certain  epigenetic  formations  which  are  known  as 
"  bedded  veins,"  and  of  which  some  account  is  given  in  the 
sequel.  Usually,  however,  a  bedded  ore  is  not  so  sharply 
marked  off  from  the  schists  among  which  it  lies,  as  is  the 
case  with  a  true  vein.  The  bedded  ore  does  not  traverse 
overlying  and  underlying  schists,  nor  does  it  send  out  veins. 
It  behaves,  in  short,  like  a  truly  contemporaneous  bed — 
following  all  the  flexures,  folds,  and  crumplings  of  the  series 
in  which  it  occurs.  The  thickness  of  such  beds  varies 
indefinitely :  they  are  usually  lenticular,  and  thicken  and 
thin  out  irregularly.  From  a  few  inches,  they  may  gradually 
swell  out  to  many  feet  or  even  yards.  The  thickest  bed  of 
ore  yet  encountered  is  that  of  Grangesberg  in  Sweden,  which 
is  not  less  than  100  yards.  In  that  country  the  bedded  ores 
are  usually  more  or  less  closely  associated  with  crystalline 
limestone,  or  with  a  rock  consisting  mainly  of  pyroxene  and 
amphibole,  and  often  containing  garnet  and  epidote.  Iron- 
ores  are  obtained  from  similar  schistose  rocks  in  Norway. 


ORE-FORMATIONS  235 

In  Dunderlandstal,  for  example,  they  occur  in  numerous  beds 
(sometimes  as  many  as  500),  rapidly  interstratified  with  mica-schist, 
and  closely  associated  with  massive  beds  of  crystalline  limestone  and 
dolomite,  from  which,  however,  they  are  always  separated  by  a  variable 
thickness  (i  to  10  metres)  of  mica-schist  (Fig.  84).  This  ore  belt  has 
been  followed  continuously  for  a  distance  of  several  miles.  It  varies 
much  in  width,  sometimes  showing  a  thickness  of  30  to  60  metres,  and  even 
exceptionally  75  to  100  metres,  but  more  usually  ranging  between  3  to  10 
metres.  The  ore  is  a  fine-grained  mixture  of  specular  iron,  magnetite, 
and  quartz,  with  various  silicates— the  proportion  of  specular  iron  being 
double  that  of  magnetite.  Usually  the  iron-ore  is  scaly,  and  has  the 
character  of  an  iron-mica-schist.  The  minerals  associated  with  this  ore 
are  chiefly  epidote,  garnet,  and  hornblende,  also  a  little  mica,  felspar, 
etc.,  together  with  scattered  microscopic  granules  of  calcite.  It  may  be 
added  that  the  ore  contains  small  percentages  of  manganese  and 
phosphoric  acid  (  =  about  i  per  cent,  apatite).  According  to  Professor 
Vogt,  these  remarkable  ore-beds  are  undoubtedly  interstratified  with  the 


FIG.  84. — SECTION  ACROSS  ORE-BEARING  SCHISTS,  URTVAND  IN  DUNDER- 
LANDSTAL, N.  NORWAY.    (After  J.  H.  L.  Vogt.) 
S,  schists;  L,  limestone;  O,  bands  of  iron-ore  with  intervening  schists. 

schists,  and  occupy  a  definite  geological  horizon  in  the  series.  Through- 
out their  whole  extent  they  have  a  similar  chemical  and  mineralogical 
constitution.  They  have  no  genetic  connection  with  plutonic  intrusive 
masses — the  schists  among  which  they  occur  are  the  result  of  regional, 
not  of  thermal  or  contact,  metamorphism.  The  mica-schists  are  obviously 
metamorphosed  sedimentary  rocks — clay-slates  or  shales ;  the  ores,  on 
the  other  hand,  which  are  always  sharply  marked  off  from  the  schists, 
could  not  have  been  originally  mechanical  sediments  of  quartz-sand  and 
magnetite-specular-iron-sand,  seeing  that  they  contain  a  somewhat 
constant  and  relatively  high  percentage  of  phosphoric  acid.  Professor 
Vogt  has  no  doubt,  therefore,  that  the  ores  were  originally  chemical 
precipitates,  probably  formed  much  in  the  same  way  as  the  iron-ores  now 
accumulating  in  many  lakes  and  bogs.  This  explanation  is  in  keeping 
with  the  frequent  occurrence  of  petroleum,  mineral  pitch,  and  anthracite 
in  the  schistose  rocks  with  which  such  ore-beds  are  associated.  Further, 
the  lenticular  form  assumed  by  many  of  the  ore-formations  is  possibly 
suggestive  of  their  deposition  in  lacustrine  hollows.  The  frequent 
occurrence  of  phosphoric  acid  and  manganese  in  the  ores  are,  according 
to  Vogt,  readily  accounted  for.  Just  as  the  iron  must  have  been  derived 
from  the  breaking-up  of  iron-rich  minerals  (augite,  hornblende,  etc.),  so 


236 


STRUCTURAL  AND  FIELD  GEOLOGY 


the  phosphoric  acid  would  be  obtained  from  apatite  (common  as  a 
constituent  of  igneous  rocks)  and  the  manganese  from  many  different 
rock-forming  minerals.  We  may  suppose  that  regional  metamorphism 
would  bring  about  marked  changes  in  the  original  chemical  and 
mechanical  sediments.  What  were  at  first  carbonates  and  hydrates  of 
iron  would,  under  the  influence  of  heat  in  the  presence  of  moisture, 
eventually  be  transformed  into  specular  iron  and  magnetite,  while  the 
clays  associated  with  them  would  be  changed  into  mica-schist. 

Beds  of  magnetite  and  specular  iron  are  associated  with 
schistose  rocks  in  many  other  countries,  as  in  S.  Russia,  in 
the  Riesengebirge,  in  Spain,  in  the  United  States  (New 
York,  New  Jersey,  Carolina,  Michigan),  and  elsewhere. 
Carbonate  of  iron  (siderite)  is  another  ore  met  with  amongst 
schistose  rocks.  At  Huttenberg,  in  Carinthia,  it  occurs  in 
crystalline  limestones  which  are  interstratified  with  gneiss 
and  mica-schist  (see  Fig.  85).  Manganese  ores  likewise 


FIG.  85.— SECTION  ACROSS  THE  ORE-BEARING  ROCKS  OF  HUTTENBERG  IN 

CARINTHIA.    (After  F.  Seeland.) 
sch,  schistose  rocks  ;  I,  limestones  ;  o,  bands  of  iron-ore. 

occur  under  similar  conditions  in  Sweden,  Bukowina,  Brazil, 
and  the  United  States  of  N.  America. 


H._EPIGENETIC   ORE-FORMATIONS 

The  formations  included  under  this  head  are  of  later  age 
than  the  rocks  with  which  they  are  associated  or  in  which 
they  occur.  They  have  been  subsequently  introduced  into 
the  positions  they  now  occupy,  and  thus  a  large  number 
appear  in  fissures  and  other  cavities  in  rocks  of  all  kinds, 
while  in  many  cases  they  replace  pre-existing  minerals  and 
rock-masses.  They  may  be  grouped  as  fissure-veins  or  lodes, 
bedded  veins,  and  irregular  formations.  This  is  not  a  very 
satisfactory  classification,  for  one  and  the  same  ore-deposit 
may  assume  many  different  forms  in  its  course,  appearing 
sometimes  as  a  "  lode,"  sometimes  as  a  "  bedded  vein,"  or  as 


ORE-FORMATIONS 


237 


one  or  other  of  the  "  irregular  formations."  Nevertheless,  the 
classification  here  adopted  serves  to  bring  prominently  into 
view  the  various  conditions  under  which  the  epigenetic  ore- 
formations  occur. 


i.  FISSURE  VEINS  OR  LODES 

Nature  of  Fissures — An  ore-vein  or  lode  may  be  defined 
as  a  rent  or  fissure  filled  with  metalliferous  and  other  minerals 
alone,  or  with  rock-debris  in  addition.  The  fissures  in  which 
true  lodes  occur  are  often  mere  chinks  or  wider  clefts,  along 
which  no  rock-displacement  may  have  been  effected.  Narrow 
fissures  of  this  kind  may  occur  singly,  but  often  quite  a  large 
number,  occupying  parallel  or 
nearly  parallel  positions,  traverse 
the  rocks  in  some  given  direction. 
None  of  these  may  show  slicken- 
sides  or  yield  any  evidence  of 
slipping  or  faulting.  Not  infre- 
quently, however,  the  fissures 
occupied  by  lodes  are  faults, 
although  it  would  seem  that  the 
amount  of  rock-displacement  (when 
that  can  be  measured)  is  seldom 
very  great — not  often  exceeding 
two  or  three  hundred  feet,  and 
being  usually  much  less.  Many  ore- 
bearing'  faults,  however,  traverse 
highly  disturbed  and  schistose  rocks,  and  the  amount  of  dis- 
placement in  such  cases  must  be  quite  conjectural.  Be  that 
as  it  may,  it  would  appear  that  the  larger  dislocations  occur- 
ring in  a  region  rich  in  lodes  are  seldom  ore-bearing.  The 
faults  occupied  by  lodes  may  be  normal  or  reversed.  Few 
lodes  are  quite  vertical,  but  the  great  majority  approach 
verticality — the  inclination  from  the  vertical  being  termed 
the  hade  or  underlie.  The  rocks  traversed  by  a  lode  are 
known  as  the  country  or  country-rock ;  and  the  wall  of  the 
fissure  which  overhangs  the  miner  when  standing  upright  is 
termed  the  hanging-wall ;  while  that  on  which  he  stands  is 
the  footiuall  (see  Fig.  86). 


FIG.  86. — FISSURE-VEIN  OR 
LODE. 


238  STRUCTURAL  AND  FIELD  GEOLOGY 

Width  and  Extent  of  Lodes. — Individual  lodes  often 
vary  much  in  width — the  walls  of  the  fissure  approaching 
and  receding  in  an  irregular  manner,  and  now  and  again 
being  in  close  apposition,  in  which  case  the  lode  is,  of  course, 
"nipped  out."  Such  irregularities,  it  need  hardly  be  said, 
are  due  to  the  character  of  the  original  fissure,  except  when 
limestone  forms  the  wall  or  walls  of  a  lode.  In  such  cases 
the  irregular  width  of  the  cleft  has  not  infrequently  been 
caused  by  the  unequal  dissolution  of  the  rock.  Some  lodes 
are  very  narrow — a  few  feet  or  less — others  may  exceed  100 
feet  in  width.  In  the  case  of  very  broad  lodes,  however 
(say,  from  20  feet  to  100  feet),  it  must  be  understood  that 
this  is  not  the  actual  width  of  the  original  fissure,  but  includes 
as  much  of  the  adjacent  rock  as  contains  ore  in  payable 
quantity — whether  occurring  as  impregnations  or  as  strings, 
threads,  veinlets,  and  flats  (see  pp.  252,  255).  Some  broad 
lodes,  for  example,  consist  of  more  or  less  numerous  and 
approximately  parallel  veins,  occupying  very  narrow  fissures 
or  mere  cracks,  which  in  the  central  portion  of  the  "lode" 
are  often  less  than  an  inch  apart,  but  become  more  widely 
separated  towards  the  limits  of  the  fissured  area.  Lodes  of 
this  kind  are  known  as  "  sheeted  zones,"  and  sometimes 
attain  a  width  of  100  feet  or  more.  A  "sheeted  zone," 
therefore,  is  simply  a  belt  of  highly  fissured  rock,  which, 
when  gold  is  present  in  the  fissures,  may  be  profitably 
extracted  so  long  as  the  veins  are  rich  enough  or  sufficiently 
numerous. 

Lodes  differ  considerably  in  length  or  lateral  extent. 
Some  die  out  in  much  less  than  a  mile,  while  others  have 
been  followed  for  great  distances.  Probably  the  longest 
known  is  the  auriferous  "  Mother  Lode  "  of  the  Sierra  Nevada, 
California,  which  runs  in  a  relatively  straight  line  for  more 
than  70  miles. 

The  longest  veins  seem  usually  to  have  the  greatest 
vertical  range.  Some  of  these  have  been  followed  to  depths 
not  far  short  of  3000  feet,  without  showing  any  appearance 
of  dying  out.  Many  of  the  shorter  veins  wedge  out  down- 
wards or  upwards.  Lodes  of  this  kind  are  frequently  very 
irregular — branching  often  in  many  directions.  Some  wedge 
out  simply ;  others,  again,  divide  into  two  or  more  smaller  and 


ORE-FORMATIONS 


239 


gradually  diminishing  veins ;    or  they  may  break  up  into  a 
perfect  network  of  strings  and  veinlets. 

The  actual  depth  to  which  the  most  persistent  lodes  may  descend  is 
not  known.  From  several  considerations,  however,  it  may  be  inferred 
that  the  fissures  occupied  by  lodes  must  in  many  cases  have  traversed  a 
very  great  thickness  of  rock.  Since  those  fissures  were  formed  there  has 
been  excessive  denudation,  whereby  a  thickness  of  rock,  to  be  measured 
in  some  cases  by  thousands  of  feet  or  even  of  yards,  has  been  removed. 
Hence  the  present  surface  where  such  lodes  crop  out  is  very  far  below 
the  surface  that  existed  when  the  fissures  were  being  filled  with  their 
ore-formations.  In  the  case  of  certain  lodes  which  have  been  mined  to 
a  depth  of  1000  yards,  it  has  been  estimated  that  the  original  surface  may 
have  been  2000  or  even  4000  yards  higher  than  the  present,  which  would 
give  an  original  vertical  range  of  3000  or  5000  yards  at  least  for  the  lodes. 

It  is  doubtful,  according  to  some,  whether  any  cavities  could  be 
formed,  or,  having  been  formed,  could  remain  open  under  the  enormous 
pressure  of  15,000  feet  of  rock.  Others,  again,  have  questioned  the 
possibility  of  chemical  precipitations  from  aqueous  solutions  taking  place 
at  such  depths,  where  the  pressure  must  be  excessive  and  the  temperature 


FIG.  87.— SIMPLE  LODE  SHOWING  MASSIVE  STRUCTURE. 

considerably  above  that  of  boiling  water.  But,  as  Vogt  has  pointed  out, 
mineral  deposits  have  certainly  been  made  from  solutions  at  a  much 
higher  temperature  than  is  likely  to  obtain  at  a  depth  of  15,000  feet 
below  the  surface.  He  instances  the  occurrence  of  cassiterite,  topaz, 
tourmaline,  apatite,  and  other  minerals  in  the  pegmatite-veins  of  granite, 
which,  having  been  abstracted  from  the  granite  magma,  must  have  been 
deposited  from  solutions  at  a  higher  temperature  than  the  critical 
temperature  of  water — 690°  F.  or  thereabout. 

As  vertical  fissures  of  all  kinds  tend  to  die  out  upwards,  while  the 
amount  of  displacement  caused  by  faults  diminishes  in  the  same  direction, 


240 


STRUCTURAL  AND  FIELD  GEOLOGY 


(After 


we  may  infer  that  many  fissures  may  never  have  reached  the  surface  at 
the  time  of  their  infilling,  the  outcrops  we  now  see  having  been  exposed 
by  denudation. 

Simple  and  Complex  Lodes. — A  lode  is  said  to  be  simple 
when  it  occupies  one  single  well-defined  fissure  (see  Fig.  87). 

Often  enough,  however,  the 
formation  of  a  fissure  has 
been  accompanied  by  much 
rock-shattering,  the  adja- 
cent rocks  being  confusedly 
jumbled  and  crossed  in 
every  direction  by  nume- 
rous branching  cracks  and 
crevices.  When  all  these 
cavities  are  filled  with 
mineral  matter  we  have  a 
complex  lode  (see  Fig.  88). 
One  and  the  same  lode 
may  be  simple  in  one  part 
of  its  course  and  complex 
in  another.  This  is  not 

infrequently  the  case  when  a  lode  traverses  rocks  of  very 
different  kinds.  For  example,  a  vein  may  be  simple  while 
passing  through  rocks  which  have  yielded  readily  to  tension, 
but  becomes  complex  when 
it  begins  to  traverse  some 
massive  irregularly  jointed 
rock  (see  Fig.  89). 

Transverse  and  Coin- 
cident Lodes. — Lodes  cut- 
ting through  stratified  rocks 
usually  cross  the  planes  of 
bedding  at  an  angle,  and 
are  then  said  to  be  trans- 
verse (see  Fig.  90).  Now 
and  again,  however,  especi- 
ally when  the  strata  dip  at 
a  high  angle,  a  lode  may  coincide  with  the  planes  of  bedding. 
The  epigenetic  character,  however,  is  usually  apparent,  the 
lode  not  being  strictly  confined  between  two  bedding-planes, 


FIG.  88.— COMPLEX  LODE. 
R.  Beck.) 


FIG.  89. — LODE  DIVIDING  AND  BRANCHING 
IN  IGNEOUS  ROCK.    (Plan.) 


ORE-FORMATIONS 


241 


but  here  and  there  invading  both  overlying  and  underlying 
strata  (see  Fig.  91). 


FIG.  90. — TRANSVERSE  LODE.    (After  R.  Beck.) 

Systems  of  Lodes. — While  lodes  often  occur  singly,  it 
is  more  frequently  the  case  that  several  or  many  are 
associated  so  as  to  form  one  or  more  systems.  Their  general 
disposition  recalls  that  of  the  basalt-dykes  described  in 
Chapter  XIV.  Like  these,  they  trend  in  certain  definite 
directions,  some  appearing 
in  true  faults,  others  in 
simple  rents  or  fissures. 
In  certain  regions  only  one 
such  system  may  be  pre- 
sent; in  other  places  two 
or  more  systems  may 
appear,  one  set  crossing 
another.  As  the  fissures 
and  faults  in  which  they 
occur  are  the  result  of 
crustal  movements,  it  is 
not  surprising  that  groups 
of  parallel  lodes  should 

often  bear  a  definite  relation  to  the  principal  folds  and  flexures 
of  a  region.  Some,  therefore,  coincide  with  the  average  strike 
of  the  country-rock,  while  others  traverse  the  strike  more  or 
less  at  right  angles.  In  mountain  tracts  lodes  not  infre- 
quently run  parallel  to  the  general  axis  of  elevation.  Hence, 
if  the  date  of  the  elevation  be  known,  the  age  of  the  faults 

Q 


FIG.  91. — COINCIDENT  LODE.    (After 
R.  Beck.) 


STRUCTURAL  AND  FIELD  GEOLOGY 


and  fissures  is  at  once  determined.     As  crustal   movements 
have  often  affected  the  same  area  at  different  periods,  and 

not  infrequently  in  different 
directions,  new  systems  of 
divergent  and  intersecting 
folds  and  fissures  have  suc- 
cessively been  produced,  the 
relative  age  of  which  can 
usually  be  ascertained  by 
observing  the  behaviour  of 
one  system  to  another. 

Branching  and  Intersec- 
tion of  Lodes.  —  Lodes  not 
infrequently  divide  into  two 
or  more  branches,  which,  after 
pursuing  separate  courses  for 
longer  or  shorter  distances, 


AND 


again  come  together  (see  Fig. 
92).  Occasionally,  also,  two  lodes  may  gradually  converge 
until  they  meet,  and  then,  after  running  side  by  side  for  some 


FIG.  93.— LODES  CONVERGING  AND  DIVERGING. 

little  way,  may  again  diverge  (see  Fig.  93) ;  or,  instead  of 
diverging,  one  may  intersect  the  other  without  displacing 
or  shifting  it,  and  subsequently  resume  its  original  direction 


ORE-FORMATIONS 


243 


(see  Fig.  94).     In  such  a  case  the  fissure  occupied   by  the 
intersecting  and  therefore  younger  vein  is  obviously  a  simple 


FIG.  94.— LODES  CONVERGING  AND  INTERSECTING. 
rent  and  not  a  true  fault.     Occasionally,  two  veins  meet  at 


FIG.  95 — LODES  INTERSECTING  AT  RIGHT  ANGLES  WITHOUT 
DISPLACEMENT.    (Plan). 

approximately  right  angles,  the  younger  similarly  intersect- 
ing the   older  without   displacing   it   (see   Fig.   95).      Very 


244 


STRUCTURAL  AND  FIELD  GEOLOGY 


rarely  two  fissures  intersecting  at  right  angles  have  received 
their  mineral  contents  at  the  same  time  (see  Fig.  96). 


FIG.  96.— CONTEMPORANEOUS  CROSS-VEINS.    (Plan). 

Heaving  of  Lodes. — When  the  intersecting  lode  occupies 
a  fissure  of  displacement  or  true  fault,  it  invariably  shifts  or 

heaves  the  lode  it  traverses 
(see  Fig.  97).  If  the  fault  be 
normal,  then  the  older  lode 
is  shifted  in  the  direction  of 
the  downthrow ;  in  the  case 
of  a  reversed  fault  the  older 
lode  will,  of  course,  be  heaved 
in  the  opposite  direction. 

Contents  of  Fissure 
Veins. — These  are  known  as 
veinstone^  veinstuff,  matrix,  or 
gangue,  and  consist  largely  of 
crystallised  minerals,  such  as 
quartz,  calcite,  and  other  car- 

FIG.   97—HEAVING  OF  ONE  VEIN  BY      bonates  (dolomite,  magliesite, 
ANOTHER.  . ,      „ 

etc.),  barytes,  and   fluor-spar. 

Fragments  of  the  "country"  (i.e.  the   rocks  traversed  by  a 
lode)   frequently   appear,   and    often    constitute    the    larger 


ORE-FORMATIONS  245 

portion  of  the  veinstuff.  The  fragments  are  of  all  shapes — 
angular,  subangular,  or  rounded— and  some  of  them  may  show 
smoothed  and  striated  surfaces.  They  vary  also  in  size,  from 
large  blocks  down  to  finely  comminuted  particles.  The  ores 
are  irregularly  distributed  through  the  veinstone  as  grains, 
crystals,  patches  or  bunches,  laminae,  threads  and  strings, 
often  crossing  and  recrossing.  Sometimes  they  assume  the 
form  of  vertical  or  steeply  inclined  columnar  or  chimney-like 
aggregates,  surrounded  on  all  sides  by  lean  or  barren  vein- 
stuff.  Such  rudely  columnar  ore-bodies  are  known  as  shoots. 
Or  they  may  appear  in  the  form  of  more  or  less  regular 
plates  and  tabular  sheets,  disposed  in  parallel  positions  with 
similar  plates  of  veinstone ;  or,  again,  they  may  occur  as 
massive  aggregates  occupying  the  whole  fissure  to  the  exclusion 
of  any  veinstuff.  On  the  other  hand,  ore  may  be  entirely 
wanting  in  some  parts  of  a  lode,  the  fissure  being  either  filled 
with  veinstone  and  rock-rubble  only,  or  closed  by  the  apposi- 
tion of  its  walls. 

Structure  of  Fissure  Veins. — (a)  MASSIVE  STRUCTURE. 
When  a  fissure  is  entirely  filled  with  ore,  or  with  crystallised 
or  cryptocrystalline  mineral  matter  containing  ore  dissemi- 
nated through  it,  the  structure  is  said  to  be  massive  (see 
Fig.  87,  p.  239).  Galena  (lead-ore),  for  example,  is  often  met 
with  completely  filling  fissures — crystallised  veinstone  being 
either  entirely  absent  or  occurring  only  as  small  inclusions  in 
the  ore,  or  as  a  meagre  interrupted  layer  lining  the  walls. 
Auriferous  quartz-veins  are  an  example  of  the  same  struc- 
ture— the  ore  in  this  case  being  included  in  the  quartz  which 
wholly  fills  the  fissure. 

(£)  PLATY,  LAMELLATED,  OR  BANDED  STRUCTURE. — In 
this  structure  the  ore  and  the  veinstone  are  disposed  in  more 
or  less  sharply  defined  sheets  or  layers  parallel  to  the  walls 
of  the  fissures  (Plate  L.  i).  This  is  the  commonest  kind  of 
structure  met  with  in  lodes.  The  sheets  are  of  very  vari- 
able thickness,  and  may  be  few  in  number,  in  which  case 
some  or  all  may  be  relatively  thick ;  or  they  may  be 
numerous  and  all  extremely  thin — mere  laminae  of  irregu- 
larly alternating  ore  and  veinstufif.  Now  and  again,  how- 
ever, they  are  arranged  symmetrically  in  pairs.  The  opposite 
walls  may  each  be  lined,  for  example,  with  a  layer  of  quartz 


246 


STRUCTURAL  AND  FIELD  GEOLOGY 


or  other  veinstone ;  to  this  may  succeed  two  bands  of  ore, 
one  on  either  side — and  such  duplication  may  be  repeated 
again  and  again,  until  the  fissure  is  completely  filled  (Fig.  98). 

Such  a  banded  lode  is  said  to 
be  symmetrical.  The  crystal- 
lised minerals  are  often  pris- 
matic— their  longer  axes  being 
perpendicular  to  the  walls  and 
their  pyramidal  terminations 
directed  towards  the  centre  of 
the  vein.  A  section  across 
such  a  sheet  has  suggested  to 
mining  folk  its  resemblance  to 
a  comb,  and  thus  we  have  the 

term    comby    lode    applied    to 
FIG.  98.— LAMELLATED  LODE  WITH  .     ,  ..  *T        _ 

DRUSES,  symmetrical  fissure-veins.    Fre- 

quently,  the   fissures    are    not 

completely  filled — medial  cavities  of  less  or  greater  extent 
being  left.  These  are  termed  vughs  or  druses,  and  are 
usually  lined  with  crystallised  minerals. 

(c)  BRECCIATED  STRUCTURE. — Some  lodes  are  largely 
brecciated — abundant  fragments  of  mineral  plates  or  lamellae, 
together  with  pieces  of  the  country-rock,  being  scattered 
through  amorphous  or  ir- 
regularly crystallised  vein- 
stuff.  This  structure  shows 
that  the  fissure  occupied  by 
a  banded  lode  has  been 
subsequently  reopened  — 
the  crustal  movement  re- 
sulting in  the  fracturing 
and  shattering  of  the  platy 
layers  of  the  original  lode 
and  the  introduction  into 
the  reopened  fissure  of 
fragments  of  the  country-  FIG.  99.— BRECCIATED  LODE. 

rock.         Later     on,     this 

jumbled  mass  has  been  permeated  by  metalliferous  and 
mineral  solutions,  which  have  bound  the  debris  together  (see 
Fig.  99  and  Plate  L.  2).  Now  and  again  these  subsequently 


[To  face  page  24(5. 


ORE-FORMATIONS 


247 


introduced  ores  and  crystalline  minerals,  in  place  of  being 
diffused  through  the  debris,  are  found  encrusting  the  em- 
bedded pieces  of  country-rock  and  fragments  of  older  vein- 
stone with  successive  layers,  forming  what  are  termed  ring 
ores  or  cockade  ores  (Cocardenerze). 

In  some  reopened  and  refilled  veins  the  products  of  the 
first  infilling  have  not  been  entirely  broken  up — the  fissure 
has  simply  been  widened  and  a  new  comby  lode  has  been 
formed  outside  of  and  parallel  to  the  original  symmetrical 
lode.  This  reopening  and  refilling  process  has  in  certain 
cases  been  repeated  several  times,  the  lode  consisting  of  a 
succession  of  duplicate  sheets,  each  two  or  more  bands 
representing  a  separate  infilling  (see  Fig.  100).  Many  other 


III 


.   FIG.  100. — REOPENING  AND  REFILLING  OF  VEINS. 

I— I,  2,  I— 1,  2,  3,  4,  first  infilling;  IT— 1',  1',  2',  second  infilling. 

structures  may  be  observed  in  reopened  fissure-veins. 
Occasionally,  the  new  cavities  are  crowded  entirely  with 
rock-debris,  which  may  or  may  not  be  ore-bearing.  Now 
and  again,  however,  the  interstices  and  wider  spaces  between 
some  of  the  larger  blocks  detached  from  the  walls  have  not 
been  completely  filled  with  new  mineral  matter.  In  such 
cavities  (or  vughs)  finely  crystallised  minerals  and  stalactitic 
formations  frequently  appear. 

Outcrop  of  Lodes. — The  line  along  which  a  lode  comes 
to  the  surface  is  variously  termed  outcrop,  outgoing,  or  back. 
When  a  lode  consists  of  more  durable  ingredients  than  the 


248  STRUCTURAL  AND  FIELD  GEOLOGY 

rock  it  traverses,  as  is  frequently  the  case  with  quartzose  lodes, 
it  projects  at  the  surface,  and  forms  what  miners  term  a  reef. 
On  the  other  hand,  should  a  lode  be  less  durable  than  the 
country-rock,  its  outcrop  is  revealed  by  a  trench-like  depres- 
sion. Lodes,  however,  are  often  concealed  underneath  super- 
ficial deposits.  Some,  again,  do  not  reach  the  surface — either 
owing  to  the  dying-out  of  the  fissures,  or  to  the  subsequent 
accumulation  above  the  country-rock  of  later  sedimentary  or 
igneous  formations.  It  is  probable,  indeed,  that  lodes  exist 
in  many  unsuspected  places — more  particularly  in  regions 
where  considerable  unconformities  occur.  They  are  met 
with  traversing  rocks  of  all  ages — Palaeozoic,  Mesozoic,  and 
Cainozoic  alike ;  but,  as  might  have  been  expected,  are 
of  more  frequent  occurrence  in  Palaeozoic  than  in  Mesozoic, 
and  in  Mesozoic  than  in  Cainozoic  rocks.  They  are  most 
commonly  associated  with  metamorphic  rocks  and  eruptive 
masses,  although  this  is  by  no  means  invariably  the  case. 

Gossans. — A  lode  at  its  outcrop  is  usually  more  or  less 
weathered,  and  of  a  rusty  brown,  red,  or  yellowish  colour  from 
the  frequent  presence  of  ferruginous  matter.  Such  weathered 
backs  are  termed  Gossans.  The  thickness  or  depth  of  gossans 
is  quite  indeterminate.  Sometimes  they  extend  to  a  depth 
of  many  fathoms,  but  usually  they  do  not  go  much  below  the 
water-level  of  a  district.  Native  metals  (gold,  silver,  copper), 
carbonates,  sulphates,  and  phosphates  of  metals,  and  other 
metalliferous  compounds  often  occur  in  relatively  large  pro- 
portion in  gossans.  All  these  are  the  products  of  the  decom- 
position of  the  ores  of  the  original  or  unaltered  lode.  As  the 
lode  is  followed  to  greater  depths,  native  metals  and  oxidised 
ores  gradually  disappear,  and  are  succeeded  by  sulphides  or 
other  compounds  devoid  of  oxygen.  As  the  present  surface 
at  which  lodes  crop  out  must  be  far  below  that  which  existed 
at  the  time  of  their  formation,  it  will  be  readily  understood 
why  metals  such  as  gold  and  silver  should  often  occur  in 
relatively  large  proportion  in  the  gossans  of  auriferous  and 
argentiferous  lodes.  The  outcrop  of  a  vein  is  necessarily 
lowered  with  the  general  lowering  of  the  land-surface  by 
denudation.  The  chemical  action  of  percolating  water 
afTects  the  metalliferous  contents  of  the  lode,  which  ere  long 
become  oxidised,  and  may  even  be  reduced  to  the  state  of 


ORE-FORMATIONS  249 

native  metals.  These  last,  owing  to  their  superior  weight 
and  insolubility,  are  not  washed  away  with  the  lighter  and 
more  soluble  constituents,  and  thus  tend  to  become  concen- 
trated in  the  gossan.  This  is  the  reason  why  the  gossanous 
parts  of  auriferous  and  argentiferous  lodes  are  usually  richer 
than  the  underlying,  unweathered  portions.  The  richness 
of  a  gossan,  therefore,  is  apt  to  deceive  the  unwary  as  to  the 
value  of  the  subjacent  deposit — rich  gossans  having  sometimes 
been  found  capping  lodes  which  were  too  poor  to  work. 
When  a  gossan  yields  valuable  metals  or  ores,  it  certainly 
indicates  the  presence  of  these  in  the  lode  below ;  but 
whether  the  latter  is  rich  enough  to  be  advantageously 
worked  cannot  be  determined  until  the  undecomposed  material 
below  the  gossan  has  been  carefully  examined.  The  effect  of 
percolating  water  can  frequently  be  traced  to  a  considerable 
depth  below  the  "iron-hat" — or  true  gossan — the  general 
result  being  a  concentration  of  secondary  products,  consisting 
partly  of  oxides  and  partly  of  sulphides.  This  secondary 
enrichment  of  a  lode  sometimes  extends  to  a  depth  of  600 
feet  or  even  750  feet  from  the  surface. 

Association  of  Ores  in  Lodes. — The  minerals  in  lodes 
often  show  paragenetic  relations — that  is  to  say,  certain 
minerals  are  frequently  found  associated.  For  example, 
manganese-  and  iron-ores  often  occur  together,  and  the  same 
is  true  of  galena  (PbS)  and  zinc-blende  (ZnS),  of  cobalt-  and 
bismuth-ores,  and  of  cobalt- and  nickel-ores.  In  like  manner 
the  copper-sulphides  (bornite  and  chalcopyrite)  not  infre- 
quently' are  accompanied  by  iron-pyrite.  Again,  when 
bismuth  glance  (Bi2S3)  is  present,  chalcopyrite  is  seldom  or 
never  absent.  Similarly,  pyrrhotite  and  chalcopyrite  are  con- 
stant associates.  Once  more,  it  is  most  usual  to  find  fluor-spar, 
topaz,  molybdenite  (MoS2),  wolframite  [(Fe,Mn)WOJ,  and 
cassiterite  (SnO2)  occurring  together  in  the  same  ore-formation. 

Succession  of  Mineral  Deposits  in  Fissures,  etc. — It  is  not  hard  to 
understand  why  ore-deposits  should  often  seem  to  have  preferred  one 
rock  to  another.  It  is  obvious,  for  example,  that  relatively  hard,  porous, 
and  highly  fissured  rocks  would  be  more  readily  traversed  by  solutions, 
than  soft  impervious  masses,  in  which  joints  and  faults  are  apt  to  be  close, 
and  even  approximately  water-tight.  Again,  some  rocks,  particularly 
limestone,  are  more  or  less  readily  dissolved  by  acidulated  water,  and 
thus,  in  time,  yield  ample  space  for  the  deposition  of  such  ores  as 


250  STRUCTURAL  AND  FIELD  GEOLOGY 

galena  and  haematite.  Very  often,  however,  the  large  ore-bodies  occur- 
ring in  limestone,  are  simply  cases  of  metasomatic  replacement.  The 
precipitation  of  ores,  indeed,  would  seem  to  have  been  frequently  induced 
by  chemical  reaction  between  metalliferous  solutions  and  the  country- 
rock.  If  the  latter  contained  carbonaceous  matter,  for  example,  this 
would  bring  about  the  deposition  of  sulphides  from  solutions  of  metallic 
sulphates.  Precipitation  might  also  be  expected  to  occur  in  places  where 
subterranean  currents,  differing  in  temperature  and  in  the  nature  of  their 
solutions,  came  together.  Further,  in  the  case  of  ascending  currents  it  is 
obvious  that  gradually  diminishing  heat  and  pressure  must  have  played 
a  dominant  role  in  determining  the  deposition  of  substances  held  in 
solution. 

But  when  we  study  the  succession  of  minerals  in  banded  lodes,  it  must 
be  admitted  that  no  general  law  governing  that  succession  can  be 
recognised.  We  can  only  conjecture  that  the  chemical  composition  of 
the  solutions  circulating  through  the  fissures  may  have  varied  from  time 
to  time.  Sometimes  it  would  appear  as  if  successive  deposition  had 
been  determined  by  the  relative  solubility  of  the  minerals.  Frequently, 
for  example,  quartz  lines  the  walls  of  a  lode,  and  is  overlaid  by  calcite. 
Again,  it  is  highly  probable  that  the  earlier  deposits  of  ore  in  a  lode  may 
not  infrequently  have  played  the  part  of  precipitants  to  later  introductions. 
Thus  copper  solutions  might  be  reduced  by  iron-pyrite,  the  reaction 
giving  rise  to  the  formation  of  chalcopyrite.  It  is  well  known  also  that 
the  iron-pyrite  of  auriferous  quartz- veins  frequently  contains  gold.  In 
short,  it  seems  not  at  all  unlikely  that  many  of  the  common  associations 
of  ores  referred  to  above  may  be  the  result  of  one  ore  having  acted 
as  the  precipitant  of  another. 

Even  in  one  and  the  same  lode  the  mineral  succession  is  often  repeated 
several  times,  showing  that  at  intervals  similar  conditions  have  recurred 
again  and  again.  As  the  same  succession  of  minerals  may  appear  in 
lodes  filled  at  widely  separated  geological  periods,  while  lodes  of  the 
same  age  may  differ  greatly  as  regards  their  contents  and  the  order  of 
mineral  succession,  it  is  obvious  that  the  nature  and  arrangement  of  the 
ores  and  other  minerals  in  lodes  can  tell  us  nothing  as  regards  the 
geological  age  of  the  deposits. 

So  far  as  observations  have  yet  gone,  it  would  seem  that  differences 
of  depth  have  had  considerable  influence  on  the  deposition  of  minerals 
in  lodes.  In  many  regions  where  lodes  are  worked,  the  present  surface 
of  the  ground  must  be  several  thousand  feet  or  even  yards  below  the 
surface  that  existed  when  those  fissure-veins  were  filled.  In  other  cases 
we  have  no  reason  to  believe  that  any  such  excessive  denudation  has 
taken  place.  We  have  the  opportunity,  therefore,  of  studying  ore- 
deposits  which  have  been  formed  at  very  great  depths,  and  comparing 
them  with  others  of  much  less  deep-seated  origin.  Professor  De  Launay 
has  cited  quicksilver-formations  as  an  example  of  the  latter— since  they 
appear  to  be  restricted  chiefly  to  rocks  of  relatively  recent  geological  age 
which  have  been  traversed  by  eruptive  masses.  According  to  De 
Launay,  they  do  not  occur  in  regions  of  older  rocks  or  associated  with 


ORE-FORMATIONS  251 

eruptives  of  great  age,  simply  on  account  of  the  extreme  denudation 
which  those  regions  have  experienced.  The  upper  parts  of  the  older 
lodes,  which  may  have  carried  quicksilver,5have  long  since  been  removed, 
along  with  the  country-rock  traversed  by  them,  so  that  it  is  only  the 
pyritic  or  deep-seated  ore-formations  which  are  now  encountered  in  the 
lodes  of  profoundly  denuded  regions.  Again,  Professor  Vogt  has  pointed 
out  some  remarkable  differences  between  gold-,  silver-,  and  lead-bearing 
veins  of  relatively  recent  age,  such  as  those  of  Comstock,  Potosi, 
Hungary,  etc.,  and  the  much  older  lead-silver  veins  of  Norway,  Bohemia, 
the  Erzgebirge,  etc.  In  both  cases  the  lodes  are  closely  associated  with 
eruptive  rocks,  and  the  country-rock  has  undergone  much  alteration,  so 
that  the  conditions  attending  the  deposition  of  the  ores  and  veinstones 
in  all  the  regions  referred  to  appear  to  have  been  similar.  The  differences 
referred  to  by  Professor  Vogt  have  reference  not  only  to  the  contents  of 
the  lodes,  but  to  the  changes  which  have  been  superinduced  on  the 
country-rocks  ;  and  these  differences,  according  to  him,  indicate  that 
the  older  have  been  formed  at  a  greater  depth  than  the  younger  veins. 
From  his  point  of  view,  therefore,  the  latter,  if  they  were  followed  down- 
wards, would  gradually  assume  the  character  of  the  former.  Whether 
such  would  prove  to  be  the  case  is,  of  course,  conjectural,  but  Vogt's 
hypothesis  is  to  some  extent  supported  by  the  phenomena  revealed  in 
certain  deep  mines.  In  the  Cornish  mines,  for  example,  after  passing 
down  through  their  gossans  the  lodes  were  found  to  carry  copper-ore 
with  some  tin-stone ;  at  a  still  greater  depth,  a  zone  of  mixed  tin-stone 
and  copper-ore  was  encountered,  and  under  that  tin-stone  almost 
exclusively.  So,  again,  in  lodes  carrying  silver-lead-zinc  ores  it  has 
frequently  been  observed  that  the  proportion  of  zinc-blende  increases 
with  the  depth.  It  would  seem,  also,  that  in  many  manganese-iron 
formations  the  proportion  of  iron  similarly  increases  downwards.  But 
much  additional  observation  and  study  will  be  required  before  the  laws 
governing  the  genesis  and  deposition  of  ore-formations  can  be  clearly 
comprehended. 

Walls  of  Lodes. — Occasionally  the  walls  of  lodes  are 
more  or  less  slickensided — owing,  doubtless,  to  the  one  being 
ground  against  the  other.  [The  smoothed  and  slickensided 
stones  which  not  infrequently  occur  in  the  contents  of  lodes 
have  already  been  mentioned.  These  are  probably  in  some 
cases  fragments  detached  from  the  walls  after  the  latter  had 
been  smoothed  ;  in  other  cases,  they  may  have  been  slicken- 
sided in  situ>  the  blocks  and  stones  being  pressed  and  rubbed 
against  each  other  during  movements  of  the  country-rock.] 
Frequently  the  walls  of  a  lode  are  more  or  less  decomposed, 
the  width  of  decomposed  rock  being  very  variable.  Some- 
times it  may  hardly  exceed  an  inch  or  two,  while  in  other 
cases  the  rock  may  be  rotted  for  many  feet  or  yards  away 


252  STRUCTURAL  AND  FIELD  GEOLOGY 

from  a  lode.  On  the  other  hand,  the  walls  have  often  been 
rendered  excessively  hard  by  the  infiltration  of  silica.  Many 
lodes  as  they  are  followed  downwards  show  only  one  wall, 
usually  the  footwall.  In  place  of  a  definite  hanging-wall,  we 
may  have  a  considerable  breadth  of  much  shattered  and 
jumbled  rock,  the  fissures  between  the  separate  blocks  and 
fragments  being  sealed-up  with  veinstone  and  ore  (see  Fig. 
88,  p.  240).  In  other  cases  walls  may  become  obliterated, 
as  it  were,  by  the  gradual  passage  outwards  of  veinstuff  and 
ore,  which  seem  to  merge  insensibly  into  the  country-rock. 
In  yet  other  cases  no  definite  walls  can  be  traced,  and  a 
central  fissure  may  or  may  not  be  seen.  This  is  often  the 
case  with  impregnations ',  to  which  reference  will  presently  be 
made.  As  a  rule,  when  the  fissure  occupied  by  a  lode  is  a 
normal  fault,  one  or  both  walls  are  well  defined.  The  rocks 
on  the  downthrow  side  of  such  a  fault  are  often  highly 
shattered,  while  those  on  the  upcast  side  are  usually  not 
much  broken.  When  such  a  fault,  therefore,  is  subsequently 
occupied  by  an  ore-formation,  it  is  the  hanging-wall  rather 
than  the  footwall  that  tends  to  be  ill  defined.  When  a  lode 
shows  no  definite  walls,  the  original  fissure  is  more  frequently 
a  simple  rent  than  a  true  fault. 

The  ores  and  veinstones  of  a  lode  frequently  invade  the 
country-rock  not  only  as  impregnations,  but  as  sheets  (flats) 
and  subordinate  veins.  These  may  be  looked  upon  as 
merely  extensions  of  the  lode.  Sometimes  they  penetrate 
the  country-rock  along  planes  of  bedding,  of  cleavage,  or 
foliation  ;  in  other  cases,  they  obviously  follow  the  subordinate 
cracks  and  rents  which  so  often  accompany  faults. 

Stockworks. — Now  and  again  a  mass  of  rock  which  may 
consist  of  sedimentary,  of  igneous,  or  of  schistose  materials, 
may  be  very  much  jumbled  or  crushed,  and  traversed  by  an 
infinity  of  minute,  reticulating  joints  and  fissures.  This,  as 
already  mentioned,  is  not  infrequently  the  case  with  the 
country-rock  traversed  by  some  lodes.  But  highly  fissured 
rock-masses  are  not  necessarily  connected  with  great  lodes. 
Rocks  of  various  kinds  tend  to  be  more  or  less  abundantly 
jointed  while  they  are  becoming  solidified.  This  is  markedly 
the  case  with  plutonic  rocks  which  have  consolidated  from 
a  state  of  igneous  fusion.  Fissures  of  contraction  formed  in 


ORE-FORMATIONS  253 

this  way  are  liable  to  be  filled  with  subsequently  introduced 
mineral  matter.  It  has  frequently  happened,  therefore,  that 
fissured  rock-masses  have  been  permeated  by  ore-bearing 
solutions  to  such  an  extent  that  the  rock  can  be  mined  in 
successive  floors,  forming  what  is  known  as  a  Stockwork  (see 
Fig.  101).  The  infinitely  numerous  veins,  veinlets,  strings 


FIG.  ioi.— STOCKWORK. 

gn,  gneiss,  etc. ;  g,  granite. 

and  threads  of  ore  branch  and  interlace  often  in  a  most 
confused  and  irregular  manner,  although  sometimes  they 
tend  to  traverse  the  rock  in  certain  more  or  less  definite 
directions.  The  richness  of  a  stockwork  is  frequently 
increased  by  the  impregnation  of  the  rock  itself. 

General  Remarks  on  Fissure-veins. — From  what  has 
been  said  in  preceding  paragraphs,  it  will  be  gathered  that 
a  lode  may  present  many  different  features  throughout  its 
course.  It  may  be  massive  in  some  places,  banded  and 
brecciated  elsewhere.  It  may  widen  and  contract  irregularly, 
and  may  even  pinch-out  again  and  again.  At  the  same 
time  it  may  be  accompanied  by  parallel  veins  or  lodes,  some 
of  which  may  be  independent,  while  others  may  be  off-shoots 
or  branches,  which,  after  continuing  their  courses  for  longer 
or  shorter  distances,  may  again  converge  and  rejoin  the 
parent  lode ;  or,  instead  of  doing  so,  they  may  gradually  thin 
out  either  simply  or  by  subdividing  into  a  complex  of  veinlets 
and  threads.  Both  walls  may  be  well  defined  throughout; 
or  one,  usually  the  hanging-wall,  may  be  rendered  indistinct, 
either  owing  to  the  multitudinous  fissuring  of  the  rock,  or  to 
the  abundant  dissemination  of  mineral  matter  through  the 
pores  and  capillaries  of  the  "  country,"  or  to  the  metasomatic 
replacement  of  the  latter.  Or  dissemination  and  replacement 
together  may  succeed  in  obliterating  both  walls.  On  the  other 
hand,  many  lodes  are  wonderfully  regular,  continuing  between 


254  STRUCTURAL  AND  FIELD  GEOLOGY 

definite  walls,  showing  much  the  same  structure  through- 
out, and  varying  but  little  in  width  or  in  the  nature  of  their 
contents.  Lastly,  fissure- veins,  instead  of  occurring  as  more 
or  less  well-defined  lodes  following  some  determinate  direction, 
may  form  a  close  network  of  reticulating  and  intercrossing 
veinlets  and  threads,  occupying  all  the  cracks  and  crannies 
of  a  much  divided  and  shattered  rock-mass. 


CHAPTER  XVII 
ORE-FORMATIONS — continued 

Bedded  Veins  or  Quasi-bedded  Ore-Formations.  Irregular  Ore-Forma- 
tions—Masses occupying  Cavities ;  Metasomatic  Replacement ; 
Impregnations  ;  Disseminations  ;  Contact  Ore-Formations.  Origin 
of  Ore-Formations  —  Magmatic  Segregation  Ores  ;  Magmatic 
Extraction  Ores ;  Secretionary  Ores ;  Sedimentary  Ores  ;  Theories 
of  Lateral  Secretion  and  Ascension. 

2.  BEDDED  VEINS  OR  QUASI-BEDDED  ORE-FORMATIONS 

WHEN  sheets  of  ore  occur  apparently  interbedded  amongst 
more  or  less  metamorphosed  sedimentary  rocks  or  schists, 
into  which  they  send  veins  and  threads,  they  may  be  termed 
bedded  veins  or  quasi-bedded  ore-formations.  These  forma- 
tions are  not  to  be  confounded  with  the  flats  which  are 
associated  with  "masses"  (p.  259),  and  not  infrequently  also 
with  lodes  (p.  252),  for  they  are  not  connected  with  true 
fissure-veins  or  lodes.  Their  origin  is  obscure.  Not  infre- 
quently they  seem  to  occupy  planes  of  weakness  or  cavities, 
produced  during  the  process  of  folding  and  metamorphism — 
for  the  rocks  among  which  they  occur  usually  dip  at  high 
angles  and  are  more  or  less  altered.  The  veinstone  in  some 
cases  is  commonly  quartz  which  often  carries  gold  and  various 
metallic  sulphides.  While  bedded  veins  of  this  kind  are  not 
infrequently  of  considerable  width,  and  may  simulate  the 
persistence  of  true  lodes,  both  as  regards  lateral  and  vertical 
extension,  they  are  usually  more  or  less  lenticular  and  inter- 
rupted  A  good  example  is  furnished  by  the  "  saddle-reefs  " 

of  Bendigo  Goldfield  (Victoria,  Australia)  (see  Fig.  102). 
The  country-rock  at  this  place  consists  of  slaty-shales  and 
altered  sandstones,  disposed  in  a  series  of  steep  anticlines  and 
synclines.  The  abrupt  plication  of  the  rocks  has  caused 

255 


256 


STRUCTURAL  AND  FIELD  GEOLOGY 


lenticular  spaces  to  occur  between  adjoining  beds  in  the  cores 
of  the  anticlinal  and  synclinal  folds.  These  spaces  subse- 
quently filled  with  quartz  form  the  so-called  "saddle-reefs." 
Each  reef  is  thickest  along  the  middle  line  or  axis  of  a  fold, 
the  anticlinal  reefs  tapering  off  downwards,  and  the  synclinal 
reefs  upwards.  There  would  seem  to  be  a  succession  of  such 


FIG.  102. — DIAGRAM-SECTION  TO  SHOW  THE  GENERAL  STRUCTURE  OF 
"  SADDLE-REEFS." 

a,  a,  anticlines  ;  s,  s,  synclines  ;  d,  d,  dykes. 

reefs  occurring  one  above  another,  at  greater  or  less  intervals, 
the  anticlinal  reefs  being  more  frequent  and  better  developed 
than  those  occurring  in  the  synclinal  cores.  At  Bendigo, 
narrow  dykes  of  dolerite  are  associated  with  the  reefs.  The 
reefs  carry  native  gold  and  auriferous  sulphides  in  small 
grains  and  particles,  as  well  as  sharply  angular  fragments  of 
the  country-rock. 

[The  famous  Broken  Hill  silver  lode  of  New  South  Wales  is,  according 
to  Pittman,  another  example  of  a  bedded  "saddle-reef."  Broken  Hill 
itself  is  a  low  range  composed  of  various  schistose  rocks,  forming  an 
anticline,  the  axis  of  which  coincides  with  the  crown  of  the  range.  The 
back  of  the  great  saddle-reef,  before  it  was  exploited,  formed  the  crest 
of  the  range  for  a  mile  and  a  half,  but  it  has  now  been  nearly  all  quarried 
away,  the  open  cut  varying  in  width  from  twenty  to  one  hundred  feet. 
It  was  composed  mainly  of  massive  manganiferous  limonite,  yielding 
certain  percentages  of  silver  and  lead.  Throughout  this  mass  many 
cavities  (vugks)  occurred,  containing  crystals  of  carbonate  of  lead 
(cerussite),  chloride,  iodide,  and  chloro-bromide  of  silver,  and  stalactites 
of  psilomelane.  Underneath  this  gossan  or  "  iron-hat"  a  thick  zone  of 
so-called  "oxidised  ores"  and  native  silver  was  encountered,  the  zone 
yielding  variable  but  often  very  high  percentages  of  the  precious  metal. 


ORE-FORMATIONS  257 

Further  down  the  lode  was  found  to  consist  of  massive  sulphide  ore — 
an  intimate  mixture  of  argentiferous  galena  and  zinc-blende,  containing 
5  to  36  ounces  of  silver  and  2  or  3  pennyweights  of  gold  per  ton.  The 
Broken  Hill  lode  is  the  largest  of  the  kind  hitherto  encountered.  "  In 
the  widest  part  of  the  oxidised  zone  it  contained  payable  ore  for  nearly 
three  hundred  feet  in  width,  and  at  the  present  time  (1900)  the  lode  is 
being  worked  for  a  width  of  about  four  hundred  and  fifty  feet  (consisting 
of  solid  sulphide  ore)." 

If  this  great  "  reef"  really  occupies  what  was  originally  a  cavity 
formed  during  the  folding  of  the  rocks — the  crustal  deformation  could 
hardly  have  been  deep-seated,  otherwise  the  space  ought  to  have  been 
obliterated  by  the  crushing-in  of  the  compressed  rock-masses.  It  is 
perhaps  just  conceivable  that,  if  the  folding  was  a  very  protracted  process, 
ascending  ore-bearing  solutions  may  have  been  gradually  introduced, 
deposition  taking  place  part  passu  with  the  formation  of  the  cavity.  It 
is  doubtful,  however,  if  the  ore-formation  in  question  is  really  of  the 
nature  of  a  "saddle-reef."  It  ought  to  be  added  that  intrusions  of 
granite  and  diorite  traverse  the  schistose  rocks  with  which  the  silver-lode 
is  associated.] 

It  must  not  be  supposed  that  the  ore-formations  here 
described  as  "  bedded-veins  "  are  always  so  sharply  marked-off 
from  the  rocks  amongst  which  they  occur,  as  in  the  examples 
given.  Often  enough  the  ore-bed  opens  out,  as  it  were,  and 
so  shades  off  gradually  into  overlying  and  underlying  beds. 
^  In  many  cases,  indeed,  it  is  obvious  that  a  so-called  "  bedded 
vein "  or  quasi-bedded  ore-formation  is  merely  a  schistose 
rock  which  has  been  so  highly  impregnated  with  ore,  that  it 
can  be  advantageously  mined. 

It  must  be  admitted  that  it  is  often  very  difficult  to  dis- 
tinguish between  such  ore-bearing  schists  and  those  which 
have  been  described  (see  p.  234)  under  the  head  of  syngenetic 
ore-formations.  Probably  not  a  few  of  the  quasi-bedded  ore- 
bodies  associated  with  crystalline  schistose  rocks  are  largely 
of  syngenetic  origin— their  original  character  having  been 
more  or  less  obscured  by  subsequent  modifications  brought 
about  by  epigenetic  action.  Amongst  the  quasi-bedded  ores 
occurring  in  schists  are  both  oxides  and  sulphides,  but 
particularly  the  latter  —  such  as  zinc-blende,  iron-pyrite, 
chalcopyrite,  galena,  etc.  The  precise  origin  of  many  of 
these  ore-bodies,  as  already  remarked,  is  obscure.  In 
some  cases  they  may  be  the  result  of  metasomatic  action, 
and  thus  replace  pre-existing  beds.  That  many  of  the  ores, 
however,  are  true  impregnations  and  disseminations  cannot 


258 


STRUCTURAL  AND  FIELD  GEOLOGY 


be  doubted — not  infrequently  perhaps  effected  during  the 
period  of  metamorphism,  while  others  would  seem  to  have 
been  introduced  at  a  later  date. 

3.  IRREGULAR  ORE-FORMATIONS 

I.  Masses. —  The  ore-formations  grouped  under  this  head 
are  met  with  chiefly  in  limestones.  Sometimes  they  occupy 
underground  cavities — the  deserted  courses  of  subterranean 
waters — which  they  partially  or  completely  fill ;  in  other  cases 
the  ore-formation  would  appear  to  be  the  result  of  meta- 
somatic  replacement — that  is  to  say,  the  country-rock  has 
been  transformed  into  ore  by  the  more  or  less  complete 
chemical  replacement  of  its  original  constituents. 


FIG.  103. — DIAGRAM  TO  SHOW  MODE  OF  OCCURRENCE  OF  BOHNERZ., 

B,  Bohnerz  or  oolitic  limonite ;  C,  cave-earth,  etc.  ;  L,  limestone. 

(a)  Masses  occupying  Cavities. — Among  the  best  examples 
of  this  type  are  the  Bohnerz  deposits  which  are  so  frequently 
met  with  in  the  Mesozoic  limestones  of  middle  Europe. 
(Fig.  103).  Bohnerz  is  an  oolitic  or  pisolitic  limonite — the 
spherical  grains  of  which  vary  in  size  from  turnip  seeds  to  hazel- 
nuts,  and  often  show  a  concentric  radiated  structure.  The  ore 
is  usually  charged  with  many  impurities,  such  as  clay,  sand,  etc., 
and  not  infrequently  contains  fossil  organic  remains  of 
Tertiary  age — such  as  mammalian  teeth  and  bones,  together 
with  plants.  In  most  cases  the  formation  would  seem  to  be 
a  deposit  from  springs ;  but  occasionally  the  ironstone  occurs 


ORE-FORMATIONS  259 

in  the  form  of  water-rolled  fragments,  associated  with  other 
sedimentary  materials.  It  may  be  inferred,  therefore,  that 
these  have  probably  been  derived  from  some  pre-existing 
formation,  which  has  been  broken  up  at  the  surface  and  the 
debris  introduced  underground  by  the  mechanical  action  of 
water. 

Irregularly  shaped  masses  of  haematite  occurring  in  lime- 
stone may  sometimes  have  been  deposited  in  caverns  and 
underground  water-courses,  and  a  similar  origin  has  been 
assigned  to  many  analogous  masses  of  galena  and  zinc-blende 
enclosed  in  the  limestones  of  various  regions.  The  joints 
and  even  the  bedding-planes  of  a  limestone,  in  the  vicinity  of 


FIG.  104.— VEINS  IN  LIMESTONE. 

I,  limestone ;  sh,  shales ;  6,  b,  bunches ;  /,  fiat ;  g,  g,  gash-veins. 

a  "  mass,"  are  frequently  charged  with  the  same  ore,  forming 
what  are  known  as  flats,  gash-veins,  pockets,  bunches,  pipes, 
nests,  etc.  (Fig.  104).  It  is  very  doubtful,  however,  whether 
the  ore-masses  in  question  occupy  pre-existing  cavities. 
Probably  most  of  them  should  be  included  in  the  next 
group  (£). 

(b)  Masses  due  to  Metasomatic  Replacement. — This  remark- 
able change  is  well  illustrated  by  the  transformations 
undergone  by  limestone,  which  is  sometimes  replaced  by 
ores  of  iron,  lead,  zinc,  or  silver.  Some  of  the  masses  of  red 
haematite  met  with  in  the  Carboniferous  limestone  of 
Cumberland,  are  clearly  cases  of  metasomatic  replacement, 
and  possibly,  as  already  suggested,  the  same  is  true  of  them 


260 


STRUCTURAL  AND  FIELD  GEOLOGY 


all.  The  accompanying  section  (see  Fig.  105),  by  Mr  J.  D. 
Kendall,  tells  its  own  tale.  Here  the  replacement  of  the 
limestone  is  rendered  conspicuous  by  the  shaly  partings  and 
layers  which  traverse  the  haematite,  and  are  obviously 
continuous  with  the  similar  layers  and  partings  in  the  lime- 
stone at  bl.  The  limestone  has  been  transformed  into  ore, 
while  the  argillaceous  shales  (with  which  no  chemical  reaction 
could  take  place)  remain  unchanged.  There  is,  moreover,  a 
gradual  transition  from  the  haematite  into  the  limestone — the 
one  is  not  sharply  marked  off  from  the  other.  It  may  be 
added  that  the  characteristic  fossils  of  the  limestone  are  often 
partly  or  completely  changed  into  iron-ore.  Similar  pheno- 


FIG.  105. — METASOMATIC  REPLACEMENT  OF  LIMESTONE  BY  HEMATITE. 

a,  boulder-clay  ;  b,  limestone;  &i,  siliceous  ;  c,  c,  shales ;  F,  fault ;  d,  haematite  replacing  lime- 
stone ;  o,  o,  sides  of  the  open  cut.    (After  J.  D.  Kendall.) 

mena  occur  in  limestones  and  dolomites  of  various  ages 
elsewhere.  Thus,  in  Carinthia,  Triassic  calcareous  rocks  are 
metasomatically  replaced  by  ores  of  zinc,  while  in  Nevada, 
Utah,  and  other  regions  in  North  America  certain  limestones 
have  been  extensively  converted  into  silver-ores. 

2.  Impregnations. — Reference  has  already  been  made  to 
the  impregnations  which  so  frequently  affect  the  walls  of 
certain  lodes  and  the  rocks  of  a  Stockwork.  In  cases  of  this 
kind  the  ores  occur  partly  as  disseminations  (i.e.  they  occupy 
pre-existing  pores  and  interstices),  and  partly  as  metasomatic 
replacements.  For  example,  in  a  granite  impregnated  with 
tin-ore  we  frequently  find  the  ore  not  only  occupying  minute 


ORE-FORMATIONS  261 

fissures  in  the  rock,  but  here  and  there  replacing  the  felspar, 
the  form  of  which  it  retains.  It  is  this  constant  passage  of 
one  type  or  form  of  epigenetic  ore-formation  into  another 
that  makes  it  impossible  to  separate  them  into  well-defined 
or  natural  groups. 

3.  Disseminations. — Ores  are  sometimes  disseminated 
through  a  rock  in  such  a  way  as  to  show  that  they  are  not 
original  constituents  of  the  rock,  but  have  been  subsequently 
introduced ;  for  they  occupy  its  minute  pores,  interstices, 
capillaries,  and  larger  cavities.  A  good  example  of  this  kind 
of  ore-formation  is  supplied  by  the  copper-bearing  sandstones 
and  conglomerate  which  were  formerly  worked  at  Alderley 
Edge  and  Mottram  St  Andrews,  near  Macclesfield.  Green 
hydrated  copper  carbonate  (malachite)  and  the  blue  variety 
(azurite)  are  disseminated  through  the  cementing  material 
of  the  rocks,  the  constituent  grains  and  pebbles  being  in  this 
way  coated  with  ore.  Small  quantities  of  ores  of  lead, 
manganese,  iron,  and  cobalt  occur  in  the  same  sandstones. 
The  most  notable  examples,  however,  of  such  disseminations 
are  the  auriferous  conglomerates  of  the  Rand  in  Transvaal, 
S.  Africa.  The  strata  of  sandstone  in  which  the  gold-bearing 
conglomerates  occur  at  Witwatersrand,  dip  at  a  high  angle 
(60°  to  80°),  but  the  inclination  decreases  as  the  beds  are 
followed  downwards.  Gold  occurs  chiefly  in  the  siliceous 
cementing  material  of  the  conglomerates,  and  is  highly 
crystalline,  appearing  with  sharp  edges  under  the  microscope. 
In  this  respect  it  differs  from  the  gold  of  placers,  much  of 
which  shows  no  trace  of  crystalline  form.  Associated  with  it 
are  many  secondary  minerals,  such  as  pyrite,  marcasite, 
chlorite,  talc,  sericite,  etc.  The  strata  contain  no  fossils,  and 
were  probably  accumulated  in  a  lake.  Subsequently  the  whole 
series  of  deposits  were  subjected  to  crustal  movement,  being 
tilted,  compressed,  fractured,  and  faulted,  and  then  or  at  a  later 
period  were  traversed  by  dyke-like  intrusions  of  various 
igneous  rocks  (see  Fig.  106).  Concurrently  with  the  crustal 
disturbance  or  with  the  igneous  intrusions,  siliceous  and 
metalliferous  solutions  permeated  the  strata,  making  their  way 
more  readily  through  the  conglomerates  than  the^close-grained 
sandstones.  Hence  it  is  in  the  former  that  gold  and  crystallised 
minerals  occur  most  abundantly.  It  is  worthy  of  note,  however, 


262 


STRUCTURAL  AND  FIELD  GEOLOGY 


that  gold  is  confined  to  particular  beds  of  conglomerate — 
other  layers  of  the  same  kind  of  rock  containing  little  or  none. 
Possibly  this  may  be  explained  by  the  presence  of  reducing 
agents  in  the  one  case  and  their  absence  in  the  other. 
According  to  Messrs  Hatch  and  Corstorphine  it  is  difficult 
to  say  what  this  reducing  agent  was.  They  point  to  the 
frequent  association  of  gold  with  pyrite  as  suggesting  that 
the  latter  had  something  to  do  with  the  precipitation  ;  and 
they  suspect  that  the  carbonaceous  matter — plentifully  present 
in  some  richly  auriferous  portions  of  a  conglomerate — may 
have  played  a  greater  role  as  a  reducing  agent  than  is 
commonly  supposed. 


FIG.  106.— REVERSED  FAULT  IN  THE  GOLD-BEARING  ROCKS  AT  JOHANNES- 
BURG.    (After  Schmeisser.) 

s,  s,  sandstones,  etc.;  c,  c,  c,  beds  of  gold-bearing  conglomerate  (so-called  "reefs");  d,  dyke 

lying  in  fault. 

Yet  another  example  of  disseminations  may  be  given. 
At  Keeweenaw  Point,  Lake  Superior,  occur  certain  much 
decomposed  igneous  rocks  (melaphyres)  with  interbedded 
conglomerates.  Native  copper  is  found  both  in  the  con- 
glomerates and  the  igneous  rocks,  which  are  old  lavas,  the 
pebbles  of  the  former  being  often  encrusted  with  it,  while 
the  amygdaloidal  cavities  of  the  latter  are  frequently  lined 
and  occasionally  completely  filled  with  the  same  metal. 
Copper  occurs  also  in  the  joints  of  the  rocks  and  the  fault- 
fissures  traversing  the  strata,  so  that  at  Keeweenaw  Point 


ORE-FORMATIONS  263 

we  meet  with  a  union  of  at  least  two  kinds  of  ore-formation — 
disseminations  and  true  fissure  veins — both  of  which  have 
doubtless  had  the  same  origin  and  were  formed  at  the  same 
time.  The  copper  is  often  enclosed  in,  or  itself  encloses, 
zeolites,  thus  clearly  showing  it  has  been  introduced  as  an 
aqueous  solution.  The  whole  series  of  rocks,  after  having 
been  fissured  and  faulted,  has  been  acted  upon  by  hot  and 
cold  percolating  waters,  which  have  produced  much  alteration, 
probably  leaching  out  the  copper  from  the  igneous  rocks,  and 
depositing  it  where  it  is  now  found. 

The  famous  copper-slate  of  Mansfeld,  in  Thuringia,  which  has  been 
worked  for  several  centuries,  belongs  to  a  class  of  ore-formations  which 
some  have  considered  to  be  a  kind  of  connecting  link  between  epigenetic 
and  syngenetic  accumulations.  The  Kupferschiefer  (copper-slate)  is  one 
of  the  subdivisions  of  the  Permian  system  of  Germany.  The  succession 
of  deposits  in  Thuringia  being  as  follows  : — 

5.  Bunter  (sandstone,  etc.). 

4.  Zechstein  (dolomite  with  rock-salt  and  gypsum). 

3.  Kupferschiefer  (copper-slate  or  shale). 

2.  Weissliegendes  (thin  white  sandstone). 

i .  Rothliegendes  (red  sandstone  and  conglomerate). 

The  ores  occur  chiefly  in  the  Kupferschiefer  and  mostly  in  its  lower 
portion,  being  disseminated  usually  in  fine  grains  or  occurring  in  thin 
layers  and  nests.  So  abundant  is  this  fine  dust-like  dissemination  that 
the  rock  when  broken  across  gives  a  metallic  reflection  in  sunlight.  The 
most  abundant  ores  are  sulphides  of  copper,  but  associated  with  these 
occur  ores  of  silver,  zinc,  lead,  iron,  nickel,  cobalt,  etc.,  mostly  as 
sulphides  or  compounds  devoid  of  oxygen.  The  copper-slate  is  black 
and  bituminous,  not  more  than  two  feet  thick,  and  sometimes  so  hard  that 
it  rings  under  the  hammer.  It  is  often  crowded  with  fish-remains,  and 
with  relics  of  conifers,  such  as  twigs,  cones,  and  leaves,  the  fossils 
being  often  encrusted  with  or  replaced  by  ore.  The  presence  of  a 
brachiopod  (Linguld]  is  suggestive  of  the  marine  origin  of  the  shales. 
The  whole  character  of  the  strata,  however,  leads  to  the  belief  that 
deposition  took  place  in  an  inland  sea  or  salt  lake.  The  origin  of  the 
ores  has  been  much  discussed,  some  holding  that  they  are  syngenetic, 
or,  in  other  words,  chemical  precipitates.  Those  who  maintain  this  view 
are  of  opinion  that,  during  the  formation  of  the  shale-beds  the  water 
became  occasionally  habitable  and  swarmed  with  fish,  which  later  on 
were  poisoned  by  an  abundant  influx  of  water  charged  with  salts  of 
copper.  Certainly  the  fossil  fish  of  the  copper-slate  often  occur  in  bent 
and  contorted  attitudes,  as  if  they  had  been  suddenly  killed.  Similar 
appearances,  however,  are  met  with  in  deposits  which  contain  no  ores. 
In  the  Old  Red  Sandstone  of  Dura  Den,  for  example,  whole  surfaces 
of  certain  beds  were  covered  with  ganoid  fishes,  lying  in  all  directions, 


264  STRUCTURAL  AND  FIELD  GEOLOGY 

often  in  convulsed  attitudes.  The  same  phenomena  are  encountered  in 
the  famous  lithographic  limestone  of  Solenhofen,  in  the  Tertiary  deposits 
of  Monte  Bolca,  and  even  in  the  marl-slate  of  England,  which  is  on  the 
same  geological  horizon  as  the  copper-slate  of  Mansfeld,  but  which, 
unlike  the  latter,  contains  no  copper-ore.  The  sudden  descent  to  the 
sea  of  a  large  volume  of  fresh  water  sometimes  results  in  the  wholesale 
destruction  of  fishes.  Thus,  in  January  1857,  an  immense  body  of  fresh 
water,  descending  by  subterranean  courses,  was  suddenly  discharged 
upon  the  sea-floor  off  the  coast  of  Florida.  So  great  was  this  discharge 
that  the  saltness  of  the  sea  was  sensibly  diminished,  and  myriads  of  dead 
fish  floated  on  the  surface  and  were  strewn  along  the  shore.  Again, 
earthquake  shocks  have  sometimes  been  equally  destructive.  During 
the  Indian  Earthquake  of  1897,  for  example,  fishes  were  killed  in  myriads 
as  by  the  explosion  of  a  dynamite  cartridge,  and  for  days  afterwards  the 
river  Sumesari  was  choked  with  their  dead  bodies.  That  similar  results 
must  have  attended  earthquake  shocks  in  earlier  ages  cannot  be  doubted. 
As  the  sudden  destruction  and  entombment  in  mud  of  large  numbers  of 
fish  may  be  due  either  to  sudden  freshening  of  salt  water  or  to  earthquake 
shock,  it  is  obvious  that  the  abundant  fish  remains  of  the  Mansfeld 
copper-slate  cannot  be  cited  in  support  of  the  view  that  they  were 
poisoned  by  metallic  solutions.  It  would  seem  more  probable,  therefore, 
that  the  ores  by  which  they  are  encrusted  are  really  epigenetic,  or 
subsequent  introductions.  Further,  it  must  be  noted  that  the  ore- 
formations  in  question  are  not  confined  to  the  bituminous  slate,  but  are 
met  with  also  in  numerous  fissures  which  pass  upwards  into  the  overlying 
Zechstein  ;  and  it  has  been  observed  that  the  percentage  of  ore  contained 
in  the  copper-slate  itself  increases  as  it  approaches  those  fissures.  It 
is  remarkable  that  the  Permian  system,  all  the  world  over,  is  apt  to 
show  impregnations  of  copper-ore.  Towards  the  close  of  that  period 
crustal  movements  seem  to  have  affected  wide  areas,  while  volcanic  action 
was  displayed  in  many  regions.  Possibly,  therefore,  it  was  during  this 
period  of  subterranean  activity  that  the  strata  were  traversed  by  copper- 
bearing  solutions  ascending  from  below. 

4.  CONTACT  ORE-FORMATIONS 

Under  this  head  are  included  sheets,  irregular  masses, 
ramifying  veins  and  threads,  etc.,  occurring  in  rocks  usually  at 
or  near  their  junction  with  plutonic  masses.  These  ore- 
formations  (Fig.  107)  include  iron  oxides  and  sulphides,  and 
frequently  also  ores  of  copper,  lead,  zinc,  tin,  arsenic,  antimony, 
mercury,  etc. ;  and,  in  some  cases,  gold  and  silver.  They  may 
occur  at  or  very  close  to  the  junction-line — particularly  when 
the  rocks  surrounding  the  igneous  mass  are  much  broken 
and  jumbled ;  or  they  may  be  met  with  at  the  surface  for  a 
mile  or  more  away  from  an  igneous  batholith,  but  seem  never 


ORE-FORMATIONS  265 

to  stray  beyond  the  zone  of  altered  and  metamorphosed 
country-rock  which  surrounds  it.  They  may  occur  in  any 
kind  of  rock,  and  are  frequently  accompanied  by  the  "  contact 
minerals"  referred  to  in  a  previous  chapter  (p.  217).  Good 
examples  are  supplied  by  the  cassiterite-veins  which  occur 
in  genetic  connection  with  batholiths  of  acid  igneous  rock, 
and  the  apatite-veins  which  are  in  like  manner  associated 
with  masses  of  gabbro.  The  contents  of  these  veins,  as  we 
have  seen,  appear  to  have  been  extracted  from  the  still  liquid 
or  not  yet  fully  congealed  igneous  masses  and  carried  into  the 
surrounding  rocks.  They  are,  in  short,  among  the  phenomena 
of  contact  metamorphism.  According  to  Professor  Vogt, 


FIG.  107. — SECTION.    CONTACT  ORE-FORMATION  OF  GOROBLAGODAT  (URAL 

MOUNTAINS).     (After  T.  Tschernyscheff.*) 
g-r,  garnet-rock ;  p,  orthoclase-porphyry ;  o,  magnetic  iron-ore. 

who  has  made  such  phenomena  a  special  study,  many  other 
ore-formations  met  with  in  the  vicinity  of  eruptive  masses 
are  of  the  same  origin.  As  examples,  he  refers  to  the  pyritic 
deposits  .occurring  at  Sulitelma  and  other  places  in  Norway, 
at  Tharsis,  San  Domingo  and  elsewhere  in  Spain,  at  Ram- 
melsberg  in  the  Harz,  and  Schmollnitz  in  Hungary.  In  all 
these  cases  the  formatipn  of  the  ore-deposits  is  ascribed  to 
pneumatolytic  processes,  following  eruptive  intrusions.  Vogt 
further  draws  attention  to  the  fact  that  the  younger  gold  and 
silver  veins  (such  as  those  occurring  along  the  Carpathians, 
and  at  many  places  in  Colorado,  Utah,  Nevada,  California, 
Mexico,  Peru,  Bolivia,  New  Zealand,  Japan)  are  in  like 
manner  closely  associated  with  recent  eruptions  of  igneous 

*  Until  quite  recently  the  iron-ore  of  Goroblagodat  was  considered 
to  be  an  example  of  magmatic  segregation,  which  is  certainly  suggested  by 
TschernyschefPs  section.  It  is  now,  however,  believed  to  be  of  epigenetic 
origin. 


266  STRUCTURAL  AND  FIELD  GEOLOGY 

rock  of  various  kinds.  In  each  district  where  they  occur  they 
belong  to  the  latest  or  one  of  the  latest  epochs  of  volcanic 
activity  for  that  district.  Hot  springs,  solfataras,  etc.,  are 
frequently  found  near  them,  and  even  when  these  are  absent 
the  surrounding  rocks  are  always  more  or  less  altered,  as  if 
they  had  been  subjected  to  the  action  of  heated  water  and 
vapours.  The  same  authority  would  ascribe  a  similar  origin 
to  the  older  lead-silver  veins  of  the  Erzgebirge,  the  Harz, 
Kongsberg  (Norway),  Przibram  (Bohemia),  etc.,  and  the  old 
gold-quartz  "  Mother  Lode  "  in  California. 

The  ore-formations  of  Sarawak  have  been  recently  shown 
by  J.  Somerville  Geikie  to  be  true  contact-formations.  They 
include  ores  of  iron,  antimony,  arsenic,  zinc,  lead,  mercury, 
etc.,  and  native  gold  and  arsenic.  The  region  is  occupied 
chiefly  by  Mesozoic  limestone  and  shales,  which  are  often 
highly  shattered  and  brecciated  and  saturated  with  silica. 
Numerous  dykes  and  sills  of  quartz-porphyry  traverse  these 
rocks,  and  are  supposed  to  proceed  from  a  concealed  batholith 
of  granite.  Indeed,  only  a  few  miles  away  from  the  mines 
granite  comes  to  the  surface  to  form  considerable  hills.  The 
ores  occur  not  in  the  form  of  true  lodes  but  mostly  as  impregna- 
tions and  disseminations  in  the  shales,  and  as  irregular  bodies 
in  the  limestone.  Now  and  again  the  dykes  yield  a  small 
percentage  of  gold. 

Origin  of  Ore-Formations. — Here  we  refer  mainly  to  epigenetic 
formations — the  origin  of  syngenetic  ore-formations  has  already  been 
sufficiently  discussed.  We  have  learned  that  native  metals  and  ores  of 
various  kinds  occur  as  original  constituents  of  crystalline  igneous  rocks — 
the  ores  being  sometimes  so  abundantly  developed  that  they  can  be 
profitably  worked.  From  the  researches  of  Sandberger  and  others, 
moreover,  we  know  that  minute  quantities  of  many  of  the  heavy  metals 
have  been  detected  in  such  minerals  as  olivine,  augite,  hornblende,  and 
mica.  Olivine,  for  example,  has  yielded  iron,  nickel,  copper,  and  cobalt ; 
in  augite  have  been  detected  iron,  copper,  and  cobalt,  and  less  frequently, 
nickel,  lead,  tin,  and  zinc,  while  antimony  and  arsenic  are  occasionally 
present ;  from  hornblende  have  been  obtained  copper,  arsenic,  and 
cobalt,  and  not  infrequently  lead,  antimony,  tin,  zinc,  and  bismuth  ;  lastly, 
in  the  micas  (which  are  often  specially  rich  in  the  heavy  metals),  have 
been  recognised  tin,  arsenic,  copper,  bismuth,  uranium,  lead,  zinc,  silver, 
cobalt,  and  nickel.  Doubtless,  the  proportion  of  metal  present  in  any 
individual  mineral  is  extremely  minute,  but  the  sum  of  metal  contained 
in  this  way  by  the  several  constituents  of  a  rock-mass  must  really  be  very 
considerable.  From  the  phenomena  connected  with  contact  ore- 


ORE-FORMATIONS  267 

formations  there  seems  no  reason  to  doubt  that  a  molten  magma  is  not 
infrequently  rich  in  metallic  materials,  and  that  solutions  of  metal  have 
proceeded  from  many  batholiths  while  these  were  solidifying  and  cooling. 
We  seem,  therefore,  justified  in  concluding  that  igneous  rocks  are  the 
chief,  if  not  the  only,  sources  from  which  the  metals  of  most  epigenetic 
ore-formations  have  originally  been  derived. 

The  ore-formations  due  to  pneumatolytic  action  may  be  looked  upon 
as  connecting  links  between  the  truly  syngenetic  ores  of  eruptive  rocks 
on  the  one  hand,  and  the  typical  epigenetic  ore-formations  of  lodes  on 
the  other.  It  is  obvious  that  the  process  described  as  magmatic  extrac- 
tion is  closely  related  to  that  of  magmatic  segregation.  In  the  case  of 
the  latter  the  ore  has  separated  out  at  the  time  of  the  eruption  and 
consolidation  of  the  molten  magma.  They  are,  in  short,  original 
constituents  of  the  rocks  in  which  they  occur.  Contact  ore-formations 
are,  no  doubt,  also  of  igneous  derivation,  for  they  consist  of  materials 
which  have  been  extracted  from  a  molten  magma  and  carried  into  the 
country-rock  by  superheated  vapours.  They  are  thus  epigenetic — i.e.  of 
later  date  than  the  rocks  which  contain  them.  Even  in  cases  where 
such  magmatic-extraction  ores  penetrate  the  igneous  rocks  themselves  as 
veins  and  impregnations,  they  may  be  yet  described  as  epigenetic.  For, 
in  such  cases,  their  formation  must  have  been  somewhat  later  than  the 
consolidation  of  the  rock  they  penetrate.  It  is  quite  possible,  indeed, 
that  they  may  have  been  derived  from  some  still  molten  or  imperfectly 
solidified  portion  of  the  igneous  mass.  While,  therefore,  such  ore- 
formations  probably  owe  their  origin  to  solfataric  or  after-action,  and 
may  thus  be  said  to  belong  to  the  same  period  of  plutonic  action  as  the 
igneous  rock  in  which  they  appear,  yet  it  is  obvious  that  they  must  be  of 
somewhat  later  formation,  and  are,  therefore,  properly  included  in  the 
epigenetic  class. 

Concerning  the  origin  of  other  epigenetic  ore-formations,  many 
different  and  often  conflicting  views  have  been  held.  They  are  all  doubt- 
less secondary  formations,  derived  in  the  first  place  either  from  igneous 
rocks  or  from  veins,  etc.,  of  pneumatolytic  origin.  The  process  whereby 
epigenetic  ores  in  general  have  come  into  existence  might  be  shortly 
defined  as  a  process  of  concentration.  By  mechanical  and  chemical 
operations,  igneous  rocks  have  been  broken  up  and  the  resultant  products 
have  gone  to  form  sedimentary  or  aqueous  deposits  of  one  kind  and 
another.  These  last  have  in  their  turn  been  subjected  to  similar  changes 
— and  new  accumulations  have  been  built  up  out  of  their  ruins.  At  the 
surface  of  the  earth  it  is  the  mechanical  deposits — gravel,  sand,  and 
mud — which  are  most  conspicuous,  but  immense  quantities  of  materials 
are  also  carried  in  solution,  some  portion  of  which,  under  favourable 
conditions,  may  be  observed  forming  here  and  there  as  chemical  pre- 
cipitates ;  but  the  great  body  of  dissolved  mineral  matter  finds  its  way 
out  to  sea.  The  less  soluble  metals  and  metallic  compounds  derived 
from  the  disintegration  of  pre-existing  rocks  become  concentrated  in  the 
mechanical  and  chemical  sediments  now  in  process  of  formation  at  the 
surface.  This  may  be  illustrated  by  the  disintegration  of  a  basic  rock, 


268  STRUCTURAL  AND  FIELD  GEOLOGY 

such  as  basalt.  By  the  various  epigene  agents  of  change,  the  ingredients 
of  this  rock  are  converted  partly  into  relatively  insoluble  and  partly  into 
soluble  materials.  The  felspar,  for  example,  is  broken  up,  and  transformed 
into  carbonates  of  lime,  etc.,  and  hydrous  silicate  of  alumina — the  former 
being  soluble,  the  latter  insoluble.  The  augite  and  olivine,  in  like  manner, 
yield  soluble  and  insoluble  materials.  The  magnetite  and  ilmenite,  which 
are  often  abundant,  are  not  so  prone  to  alteration.  Hence  when  basalt- 
rocks  are  finally  reduced  by  epigene  action,  their  relatively  insoluble 
materials  are  represented  by  clay,  some  siliceous  sand,  and,  it  may  be, 
iron-ores — the  latter  often  becoming  mechanically  separated  in  alluvial 
deposits  as  black  "  iron-sand."  Or  the  iron  content  of  the  basalt  may  be 
largely  carried  away  in  solution  as  a  bicarbonate,  and  eventually  be 
thrown  down  as  a  chemical  precipitate.  Thus,  partly  by  mechanical  and 
partly  by  chemical  processes,  the  iron  distributed  through  the  original 
rock  as  a  primary  content  tends,  as  the  final  result  of  epigene  action,  to 
become  concentrated.  In  like  manner,  other  heavy  metals  and  more  or 
less  insoluble  ores,  derived  from  the  disintegration  of  many  different 
igneous,  schistose,  and  derivative  rocks,  and  from  the  breaking-up  of 
pre-existing  ore-formations,  are  similarly  often  concentrated  in  recent 
mechanical  and  chemical  accumulations. 

The  process  of  rock-disintegration  and  decomposition,  however,  is  not 
confined  to  the  earth's  surface,  but  affects  the  crust  at  all  depths  to  which 
water  can  descend.  The  rock-changes  produced  below  ground  are,  of 
course,  almost  exclusively  chemical.  Water  descending  from  the  surface 
often  plays  a  double  part.  It  not  only  attacks  the  rocks,  leaching  out 
their  soluble  materials,  but,  when  it  becomes  a  saturated  solution,  it  may 
redeposit  its  burden  in  the  pores,  capillaries,  and  more  open  spaces 
through  which  it  filters.  The  solvent  power  of  underground  water  is 
rendered  evident  by  the  immense  quantities  of  material  which  are 
brought  up  to  the  surface  by  springs,  and  by  the  cavities  which  result 
from  the  removal  of  all  this  soluble  rock-stuff.  It  is  further  seen  in  the 
phenomena  displayed  by  most  epigenetic  ore-formations — for  we  can 
hardly  doubt  that  the  contents  of  lodes,  etc.,  ores  and  veinstones  alike — 
have,  to  a  large  extent  at  least,  been  dissolved  out  of  the  rocks  of  the 
crust  at  all  depths  by  the  action  of  water.  The  only  conspicuous 
exceptions  among  epigenetic  ore  -  formations,  are  those  "  contact- 
deposits"  which  have  been  formed  directly  by  the  heated  vapours 
escaping  from  eruptive  masses. 

From  a  general  point  of  view,  therefore,  ore-formations  would  seem 
to  come  naturally  under  the  following  divisions  : — 

1.  Magmatic-segregation  Ore-formations. — Under  this  head  would  be 
classed   all  native  metals   and  ore-masses   occurring   as   original   con- 
stituents of  igneous  rocks. 

2.  Magmatic-extraction  Ore- formations. — This  division  would  embrace 
the  various  ore-deposits  which  are  genetically  connected  with  eruptive 
rocks,  and  are  the  result  of  pneumatolytic  processes. 

3.  Secretionary  Ore-formations. — In  this  group  would  be  included  the 
great   majority   of  ore-deposits   formed  underground  by  the   chemical 


ORE-FORMATIONS  269 

| 

action  of  circulating  water.  The  materials  of  these  formations  have 
been  derived  partly  from  molten  magmas,  and  partly  from  the  disintegra- 
tion and  decomposition  of  rock-masses  of  all  kinds,  and  have  been 
carried  in  solution  and  subsequently  deposited  as  chemical  precipitates 
in  pores  and  cavities  of  every  shape,  size,  and  origin.  The  group 
would  include  all  cases  of  metasomatic  replacement,  impregnation,  and 
dissemination. 

4.  Sedimentary  Ore-formation. — Under  this  head  would  come  all 
ore-deposits  which  have  originated  at  the  surface,  however  deeply  in 
many  cases  they  may  be  now  buried,  and  however  much  they  may  have 
been  modified.  Here  we  should  group  precipitates  from  aqueous  solution 
formed  in  lakes,  etc.,  and  clastic  ore-formations  of  every  kind,  whether 
now  occupying  a  superficial  position  or  occurring  as  beds  interstratified 
with  sedimentary  strata  of  any  age.  Many  of  the  ore-formations  truly 
interbedded  with  schistose  rocks  would  be  similarly  placed  in  this 
division. 

The  mode  of  formation  of  the  ore-deposits  included  in  groups  I,  2,  and 
4  is  sufficiently  obvious  and  need  not  be  further  discussed.  The  precise 
origin  of  many  secretionary  ore-formations,  on  the  other  hand,  is  often 
obscure,  and  has  been  a  fruitful  subject  of  controversy.  Many  different 
explanations  of  their  phenomena  have  been  advanced,  but  of  these  we 
need  only  refer  to  the  two  which  are  at  present  most  in  vogue,  namely, 
the  theories  of  (a)  lateral  secretion  and  (b)  ascension. 

It  has  long  been  noted  that  the  mineral  contents  of  a  lode  are 
not  infrequently  influenced  by  the  character  of  the  country-rock  it 
traverses.  Thus  one  and  the  same  lode  may  be  productive  while  passing 
through  some  particular  kind  of  rock,  and  unproductive  when  certain 
other  kinds  of  rock  form  its  walls.  In  Cumberland,  for  example,  the 
lead  veins  are  usually  highly  productive  when  traversing  limestone,  but 
barren  when  the  country-rock  is  slate.  So  again,  in  Derbyshire,  the  lead- 
veins  generally  carry  ore  when  the  walls  are  limestone,  while  little  or  no 
ore  appears1,  in  those  parts  of  the  lodes  which  pass  through  the  "  toad- 
stones"  (todt,  dead,  or  unproductive),  a  local  name  for  certain  more  or 
less  decomposed  igneous  rocks.  This  apparent  relation  between  lodes 
and  their  country-rock  had  been  variously  explained  before  it  began  to  be 
suspected  that  the  contents  of  the  veins  might  possibly  have  been  derived 
by  a  kind  of  lateral  secretion  from  the  adjacent  rocks.  If  the  materials 
had  originally  been  diffused  through  these  rocks,  it  was  conceivable  that 
circulating  water  might  have  leached  them  out  and  redeposited  them  in 
open  fissures,  etc.  The  researches  of  Sandberger  showed  that  this 
suspicion  or  conjecture  was  in  many  cases  at  least  well  founded,  for, 
as  mentioned  above,  he  obtained  traces  of  not  a  few  of  the  heavy  metals 
in  the  minerals  of  igneous  rocks,  and  also  in  those  of  gneiss.  He  further 
showed  that  silica,  as  well  as  lime  and  baryta,  compounds  of  which  are 
so  commonly  present  in  lodes,  might  quite  well  be  derived  from  several 
of  the  original  mineral  constituents  of  igneous  rocks.  So  that  from  the 
decomposition  of  such  rocks,  materials  for  the  formation  of  many  kinds 
of  ore  and  of  the  accompanying  veinstones  might  be  supplied.  He  put 


270  STRUCTURAL  AND  FIELD  GEOLOGY 

the  matter  to  the  proof  by  an  examination  of  the  ore-formations  and 
country-rock  of  the  Black  Forest,  and  found  that  the  phenomena  were 
in  keeping  with  his  expectations.  It  appeared  to  him  evident,  that  the 
nature  of  the  ore-formations  was  directly  affected  by  changes  or  variations 
in  the  composition  of  certain  mineral  constituents  of  the  country-rock. 
For  example,  when  the  mica  of  the  gneiss  contained  minute  proportions 
of  copper,  cobalt,  arsenic,  and  bismuth,  the  lodes  yielded  smaltite 
(arsenide  of  cobalt),  and  various  ores  of  copper.  In  other  places  where 
the  mica  of  the  country  rock  contained  silver,  arsenic,  bismuth,  cobalt, 
and  nickel,  and  little  or  no  copper,  the  lodes  were  found  to  carry  arsenical 
ores  of  silver,  cobalt,  and  nickel,  but  no  copper-ore.  Although  the 
primary  character  of  the  metallic  constituents  of  the  silicates  analysed 
by  Sandberger  has  been  questioned — the  metals  being  now  considered 
by  many  to  be  subsequent  introductions — this  does  not  quite  invalidate 
the  theory  of  lateral  secretion.  That  theory  explains  so  many  facts, 
indeed,  that  it  must  be  to  a  considerable  extent  true.  Nevertheless,  it 
is  not  a  complete  explanation,  for  it  fails  to  account  for  certain  notable 
phenomena.  If  the  contents  of  lodes  had  always  or  even  often  been 
derived  by  lateral  secretion  from  the  adjacent  country-rock,  then  the 
former  would  depend  on  the  nature  of  the  latter  to  a  much  greater  extent 
than  is  found  to  be  the  case.  Many  examples  might  be  cited  to  show 
that  there  is  no  apparent  relation  between  secretionary  ore-formations 
and  the  rocks  they  traverse.  For  example,  several  systems  of  lodes 
are  met  with  crossing  one  and  the  same  country-rock,  and  nevertheless 
carrying  very  different  assemblages  of  ores.  On  the  other  hand,  many 
lodes  cut  through  rock-formations  of  all  kinds,  igneous,  sedimentary, 
and  schistose,  without  showing  any  marked  change  in  the  nature  of 
their  contents.  Once  more,  the  opposite  walls  of  a  lode  may  consist 
of  totally  different  rocks  (schists,  it  may  be,  on  one  side,  and  greywacke, 
sandstone,  or  limestone  on  the  other),  and  yet  the  ores  and  veinstones 
may  be  symmetrically  disposed  in  corresponding  layers  on  the  two  walls. 
Such  phenomena  as  the  foregoing  occur  so  commonly  that  many 
observers  have  concluded  that  the  theory  of  lateral  secretion  must  be 
abandoned.  It  seems  to  them  more  likely  that  the  contents  of  lodes 
have  been  deposited  from  solutions  ascending  from  considerable  depths. 
We  do  not  know  to  what  depth  water  penetrates  the  earth's  crust,  but  so 
long  as  it  can  find  a  way  for  itself  there  seems  no  obvious  reason  why  it 
should  not  descend  until  it  attains  a  temperature  at  which  it  can  no 
longer  exist  as  water.  At  what  distance  from  the  surface  this  "  critical 
temperature  "  (about  690°  F.)  is  reached,  can  only  be  roughly  conjectured. 
If  the  increment  of  heat  as  observed  in  mines  and  deep  borings — 1°  F.  for 
every  50  or  60  feet  of  descent — be  continued  indefinitely  downwards,  the 
critical  temperature  for  water  would  be  reached  at  a  depth  of  over  six 
miles.  It  is  probable,  however,  that  the  rate  of  increase  observed  near 
the  surface  does  not  continue  indefinitely,  but  is  more  likely  to  diminish 
progressively  with  the  increasing  density.  If  such  be  the  case  the 
critical  point  for  water  may  not  be  reached  at  a  less  depth  than  eight 
miles  or  more. 


ORE-FORMATIONS  271 

At  a  depth  of  eight  miles  or  so  from  the  surface  it  is  hardly  possible 
that  gaping  fissures  and  cavities  can  exist.  Under  the  enormous  pressure 
at  that  depth,  the  rocks  must  be  in  a  state  of  plasticity,  and  any  open  space 
formed  during  crustal  [movements  would  very  soon  be  obliterated  by  the 
inflow  of  its  walls.  It  is  only  in  the  upper  parts  of  the  earth's  crust  that 
water  can  circulate  in  open  fissures.  This  region  has  been  aptly  termed 
by  Mr  Van  Hise  the  zone  of  fracture,  and  is  conjectured  by  him  on 
various  grounds  to  extend  from  the  surface  to  a  depth  of  about  six  miles. 
At  lower  depths  than  this  the  rocks  are  in  such  a  condition  that  even  if 
fractured  they  would  soon  be  welded  together  again — open  spaces  could 
not  exist.  In  the  zone  of  fracture,  open  fissures  may  well  extend  down- 
wards for  great  distances,  but  much  will  depend  upon  the  nature  and 
geological  structure  of  the  rocks  they  traverse.  As  these  vary  much  in 
the  resistance  they  offer  to  compression,  we  can  readily  understand  that 
one  and  the  same  fissure  may  remain  open  in  some  parts  of  its  course  and 
be  closed  elsewhere.  Many  fissure-veins,  as  we  have  learned,  show  well- 
defined  walls,  while  the  structure  of  their  included  ore-formations  leaves 
us  in  no  doubt  that  the  mineral  matter  has  been  deposited  in  what  were 
at  one  time  empty  cavities.  In  many  other  lodes  only  one  wall  is  seen, 
and  all  the  phenomena  lead  to  the  conviction  that  no  such  continuous 
cavities  existed  in  their  case — the  fissures  being  filled  up  with  crushed 
and  broken  rock,  amongst  the  interstices  of  which  the  mineral  solutions 
subsequently  made  their  way.  Again,  in  not  a  few  cases,  no  walls  to  a 
lode  are  visible — a  mere  narrow  crack  or  close  fissure  passing  through 
or  bounding  on  one  side  the  ore-bearing  rock.  In  such  a  case  the  ore- 
formation  does  not  occupy  a  cavity,  but  impregnates  the  country-rock  on 
one  or  both  sides  of  a  narrow  fissure.  All  the  phenomena  of  impregna- 
tions and  disseminations,  in  short,  show  us  that  water  makes  its  way  not 
only  along  the  various  division-planes  of  rocks,  but  soaks  more  or  less 
readily  through  the  rocks  themselves. 

It  is  not  necessary,  however,  to  suppose  that  the  water  coming  from 
plutonic  depths  is  of  meteoric  origin.  Indeed,  such  evidence  as  we  have 
would  lead  us  to  believe  that  surface-water,  in  the  paucity  or  absence  of 
open  fissures,  does  not  usually  penetrate  much  below  2000  feet.  It  is  the 
experience  of  miners  in  all  parts  of  the  world,  that  deep  mines  are 
generally  dry  and  sometimes  even  dusty.  Yet  we  know  that  when  open 
fissures  in  such  mines  are  tapped  they  not  infrequently  yield  heated 
alkaline  water.  It  is  quite  possible  that  this  water  may  originally  have 
descended  from  the  surface,  but,  on  the  other  hand,  it  may  have  come 
from  plutonic  sources.  For,  as  we  have  seen,  all  molten  rocks  contain 
vast  volumes  of  water-vapour  and  gases — to  the  action  of  which  the 
pneumatolytic  phenomena  associated  with  batholiths  are  obviously  due. 
According  to  the  theory  of  ascension,  therefore,  the  chief  agent  in  the 
formation  of  secretionary  ore-formations  is  probably  the  heated  waters 
given  off  by  plutonic  masses.  Not  only  would  these  waters  (usually 
alkaline)  carry  with  them  mineral  solutions  derived  from  the  molten 
magma,  but  as  they  continued  to  ascend  they  would  attack  the  rocks 
through  which  they  passed.  Finding  their  way  upwards  by  open  fissures 


272  STRUCTURAL  AND  FIELD  GEOLOGY 

of  all  kinds,  they  would  at  the  same  time  insinuate  themselves  into  the 
narrowest  and  closest  crevices,  and  permeate  the  pores  and  capillaries 
of  the  rocks  themselves.  The  various  mineral  constituents  of  the  rocks 
would  thus  become  altered,  and  substances  which  are  practically  insoluble 
at  the  earth's  surface  would  be  taken  up.  The  ascensionist,  therefore, 
pictures  to  himself  such  highly  heated  solutions  not  only  rising  through 
fissures,  but  being  forced  under  pressure  to  penetrate  more  or  less  deeply 
the  country-rock  on  either  side — thus  producing  the  phenomena  of 
replacement  and  dissemination.  As  the  water  ascends  to  higher  and 
higher  levels,  it  will  continue  to  deposit  mineral  matter  since  its  solvent 
power  must  become  successively  diminished  by  decreasing  temperature 
and  pressure.  The  constituents  of  the  ores  and  veinstones  formed  in  this 
way,  having  usually  been  carried  great  distances,  will  bear  no  genetic 
relation  to  the  country-rock  on  either  side  of  a  lode,  and  will  not 
therefore  be  influenced  by  the  nature  of  its  walls.  To  this  action  of 
ascending  water  we  must  add  that  of  water  descending  from  above, 
which  tends  to  dissolve  mineral  matter  from  rocks  near  the  surface,  and 
finding  its  way  into  fissures,  must  mingle  with  the  water  coming  from 
below,  and  modify  the  nature  of  the  mineral  depositions  that  take 
place. 

The  ascension  theory,  like  its  rival  the  theory  of  lateral  secretion, 
gives  a  reasonable  explanation  of  so  wide  a  range  of  phenomena  that 
it  has  met  with  much  acceptance.  The  two  theories  are  really  not 
antagonistic — the  one  merely  supplements  the  other,  although  it  must 
be  admitted  that  the  great  majority  of  ore-formations,  other  than  those 
of  sedimentary  origin  and  those  due  to  magmatic  segregation  and 
pneumatolytic  action,  are  deposits  from  heated  water  ascending  from 
plutonic  depths.  The  probabilities  are  that  the  metals  of  ore-formations 
have  been  derived  in  part  directly  from  molten  magmas,  and  in  part  by 
secretion  from  rocks  of  various  kinds,  usually  at  a  high  temperature  and 
under  great  pressure,  and  therefore  at  very  considerable  depths,  from 
which  they  have  been  carried  upwards  by  ascending  plutonic  waters.* 
Secretion,  however,  has  not  been  confined  to  great  depths,  nor  has  it 
been  effected  by  plutonic  waters  alone.  On  the  contrary,  it  must  have 
taken  place  at  many  different  levels — at  every  depth,  indeed,  to  which 
meteoric  water  can  make  its  way — and  thus  the  contents  of  lodes  have 
been  influenced  again  and  again  by  solutions  derived  from  the  country- 
rock  at  various  horizons. 

*  It  would  appear,  therefore,  that  no  hard-and-fast  line  can  be  drawn 
between  pneumatolytic  ore-formations  and  secretionary  ore-formations — 
there  will  be  a  passage  upwards  from  the  one  into  the  other. 


CHAPTER  XVIII 

GEOLOGICAL  SURVEYING 

Geological  Surveying.  Field  Equipment.  Topographical  Maps.  Data 
to  be  Mapped.  Various  Scales  of  Maps.  Signs  and  Symbols. 
Tracing  of  Exposed  Outcrops.  Tracing  of  Concealed  Outcrops — 
Evidence  supplied  by  Soils  and  Subsoils,  by  Vegetation,  by  Form 
of  Surface,  by  Springs,  by  Index-beds,  by  Alluvial  Detritus.  Carry- 
ing Outcrops  across  Superficial  Formations. 

IT  is  quite  possible  to  acquire  a  considerable  knowledge  of 
Geology  by  the  mere  intelligent  perusal  of  text-books. 
Without  having  engaged  in  practical  work,  one  may  even 
learn  to  read  a  geological  map,  and  come  to  understand  in 
a  general  way  the  structure  of  the  region  it  portrays. 
Knowledge  obtained  after  this  fashion,  however,  is  necessarily 
superficial,  and  can  never  supply  the  place  of  personal 
observation  or  study  in  the  field.  It  is  only  after  the  student 
has  familiarised  himself  with  the  phenomena  themselves,  that 
the  full  meaning  of  what  he  may  have  read  about  them  will 
dawn  upon  him.  The  best  counsel,  therefore,  which  one  can 
give  a  beginner  is  to  commence  observation  in  the  field  at 
the  earliest  opportunity,  even  before  he  has  gained  more 
than  a  mere  elementary  acquaintance  with  the  stony  science. 
Some  preliminary  knowledge  of  common  minerals  and  rocks 
is  doubtless  desirable,  and  the  student  will  be  all  the  better 
prepared  for  his  field-work  should  he  have  learned  to  recognise 
some,  at  least,  of  the  more  important  type-fossils  of  the  several 
geological  systems.  Such  elementary  knowledge,  however,  is 
not  hard  to  acquire,  and  the  want  of  it  need  not  deter  him 
from  beginning  the  study  of  rock-structure.  A  profound 
acquaintance  with  this  important  branch  of  geology  has  been 
obtained  by  several  noted  observers,  who  could  hardly  be 

s 


274  STRUCTURAL  AND  FIELD  GEOLOGY 

said    to     have    had    much    preliminary   training    in    either 
mineralogy,  petrography,  or  palaeontology. 

The  best  method  of  getting  a  grasp  of  structural  or 
tectonic  geology  is  to  attempt  the  construction  of  a  geological 
map  from  one's  own  observations.  There  are  few  more 
engrossing  or  interesting  pursuits  than  that  of  unravelling 
geological  structure,  and  the  investigator  will  find  that  the 
labour  involved  is  amply  repaid.  For  not  only  does  he  gain 
a  precise  and  intimate  knowledge  of  the  country  surveyed, 
but  he  learns  to  appreciate  geological  processes  and  their 
results  as  he  cannot  do  in  any  other  way.  His  conceptions 
of  what  is  meant  by  denudation  and  the  origin  of  surface 
features ;  of  crustal  disturbances  large  and  small ;  of  the 
metamorphism  of  rocks,  and  a  thousand  other  questions  will 
be  broad  and  assured,  or  narrow  and  uncertain,  according  as 
his  knowledge  has  been  derived  at  first  hand  from  his  own 
personal  observation  or  at  second  hand  from  books. 

Our  first  attempts  at  mapping  will  likely  enough  be 
halting  and  unsatisfactory,  but  with  zeal  and  patient  endeavour, 
experience  and  success  will  follow.  After  having  devoted 
due  attention  to  the  subject,  we  may  expect  in  time  to 
acquire  such  facility  in  reading  and  interpreting  the  stony 
record,  that  only  one  or  two  rapid  traverses  of  a  region  may 
suffice  in  many  cases  to  disclose  to  us  its  geological  structure. 
Indeed,  the  mere  configuration  of  the  ground  will  often 
reveal  to  a  trained  observer  the  leading  geological  features 
of  a  country,  and  enable  him  to  produce  a  reliable  sketch- 
map.  Experts,  however,  are  not  infallible,  and  in  rapidly 
traversing  a  region  may  miss  important  evidence  which  could 
not  have  escaped  them  had  the  ground  been  carefully 
surveyed. 

Field  Equipment. — The  apparatus  required  in  geological 
mapping  is  fortunately  neither  elaborate  nor  heavy.  There 
are  field-geologists  who  in  some  way  or  other  manage  to 
conceal  about  them  all  that  is  essential  for  the  purpose. 
Others,  again,  are  so  elaborately  accoutred  as  to  attract  the 
attention  of  every  passer-by.  The  only  necessary  apparatus, 
however,  consists  of  the  following  : — a  hammer,  a  pocket-lens, 
a  compass  and  clinometer,  a  note-book,  a  stylographic  pen,  a 
common  lead-pencil,  and  a  good,  reliable  topographical  map. 


GEOLOGICAL  SURVEYING  275 

To  these  it  is  well  to  add  a  small  protected  bottle  of  dilute 
hydrochloric  acid,  and,  of  course,  a  pocket-knife. 

The  Hammer. — In  the  selection  of  a  hammer  tastes  differ. 
For  general  purposes,  however,  that  used  by  the  officers  of 
the  Geological  Survey  can  hardly  be  surpassed.*  It  should 
not  weigh  much  over  one  pound — unless  the  observer  expects 
to  be  working  principally  among  hard  and  tough  rocks,  such 
as  granite,  gneiss,  and  schists,  when  it  may  be  desirable  to 
have  a  somewhat  heavier  implement.  The  student  will  soon 
learn,  however,  that  there  is  a  certain  art  even  in  breaking 
stones.  An  adept  by  one  dextrous  blow  with  a  light  hammer 
will  often  strike  off  a  "  specimen  "  from  some  hard,  tough 
rock,  which  a  tyro  armed  with  a  much  heavier  tool  may 
vainly  assail — all  his  efforts  resulting  only  in  the  production 
of  so  much  grit  and  powder.  There  is  some  art  not  only  in 
the  elastic  swing  of  the  arm  as  the  blow  is  delivered,  but  in 
the  selection  of  the  spot  to  be  struck,  which  will  be  determined 
partly  by  the  shape  of  the  rock-surface  and  partly  by  the 
nature  of  the  rock  itself. 

If  the  geologist  wishes  to  collect  rock-specimens  as  he 
goes  along,  a  heavier  hammer  will  be  necessary  to  detach 
fragments  of  a  sufficient  size,  besides  which  a  much  smaller 
tool  will  be  required  to  trim  the  specimens  to  the  desired 
size  and  shape.  To  these  some  geologists  add  one  or  more 
chisels,  such  as  are  used  by  masons.  These  additional 
impedimenta  may  be  carried  in  the  strong  moleskin  bag 
required  .to  hold  his  rock-specimens  and  fossils.  Heavily 
burdened  in  this  way,  however,  the  progress  of  the  hammerer 
is  apt  to  be  impeded  ;  and  if  his  chief  object  be  mapping,  he 
will  do  well  to  leave  specimen-collecting  alone  until  his 
survey  is  completed.  After  his  map  is  finished,  he  can  devote 
a  few  days  to  gathering  such  specimens  as  he  wishes  to 
procure.  As  geological  surveying  often  involves  climbing  in 
ticklish  places,  and  much  hard  walking  over  rough  ground,  it 
is  well  to  go  as  lightly  as  one  can,  if  rapid  progress  be  desired. 
A  few  capacious  pockets  to  hold  the  small  specimens  and 
chips  one  may  wish  to  examine  carefully  at  home,  will  be 
found  more  convenient  than  a  bag — the  temptation  to  fill 

*  This  hammer  is  introduced  into  many  of  the  Plates  illustrating  this 
book,  see  especially  Plates  X.,  XXXII.,  XLIX. 


276  STRUCTURAL  AND  FIELD  GEOLOGY 

which  with  choice  but  weighty  material  is  often  too  great 
to  be  resisted. 

The  Lens. — This  is  an  important  adjunct,  and  is  so  easily 
carried  that  no  field-geologist  should  be  without  it.  Even 
the  best  eyesight  may  fail  to  diagnose  the  finer  grained  rocks, 
but  there  are  few  of  these  the  character  of  which  cannot  be 
determined  by  means  of  a  lens.  For  all  ordinary  purposes  a 
lens  with  two  powers  will  be  sufficient. 

The  Compass. — This  instrument  is  primarily  used  to 
determine  the  direction  in  which  strata  are  inclined,  and  for 
this  purpose  any  pocket  compass  will  serve.  It  is  often  very 
desirable,  however,  to  take  bearings  in  order  to  fix  the  trend 
of  some  dyke,  fault,  or  other  structure,  or  to  determine  the 
exact  position  where  some  observation  is  made.  This  is 
readily  done  by  means  of  a  prismatic  compass.  An  instrument 
of  this  kind,  however,  is  seldom  required  by  the  student  who 
is  provided  with  an  accurate  large  scale  map,  such  as  the 
6-inch  map  of  the  Ordnance  Survey. 

The  Clinometer. — With  this  instrument  the  angle  of  dip  is 
measured  in  the  manner  already  described  (p.  1 27). 

The  beginner  will  probably  find  it  most  convenient  to  use 
an  instrument  in  which  compass  and  clinometer  are  combined. 
Being  the  size  of  an  ordinary  watch  it  slips  easily  into  the 
waistcoat  pocket.*  The  chief  drawback  to  this  instrument  is 
that  the  edge  which  is  to  be  held  parallel  to  the  line  of  dip  is 
too  short.  The  edge  may  be  "lengthened,"  however,  by 
placing  it  on  the  note-book,  the  hammer-handle,  or  the 
walking-stick — if  the  geologist  feels  it  necessary  to  burden 
himself  with  one.  He  will  find  ere  long  that  a  stick  is  rather 
a  hindrance  than  a  help,  and  will  probably  succeed  in  losing 
it  before  his  first  day's  work  is  done. 

The  Note-book. — This  should  not  be  too  small  nor  yet  too 
large  to  slip  into  a  side-pocke^.  A  convenient  size  is  6  inches 
by  4  inches — for  the  book  when  opened  can  then  be  used  as 
a  rough-and-ready  foot-rule  for  measuring  purposes.  The 
paper  may  be  plain  or  ruled  according  to  taste.  As  the  book 
is  meant,  however,  to  contain  not  only  notes  and  descriptions 
but  sketches  of  geological  sections,  it  is  advisable  to  have 

*  A  very  serviceable  instrument  of  this  kind  is  supplied  by  Messrs 
Troughton  &  Simms,  138  Fleet  Street,  London.     See  Appendix  E. 


GEOLOGICAL  SURVEYING  277 

some  of  the  paper  ruled  into  squares.  These  squares  may 
represent  inches,  feet,  or  yards,  and  thus  enable  the  observer 
to  sketch  on  a  correct  scale  any  rock  exposure  which  can  be 
conveniently  measured.  Until  some  facility  in  drawing  has 
been  attained,  it  is  best  to  use  first  a  common  lead-pencil, 
and  afterwards  to  ink- in  the  lines.  With  practice,  however, 
the  observer  may  eventually  be  able  to  discard  the  pencil  and 
to  sketch  directly  with  his  pen.  For  clearness'  sake  it  is  often 
advisable  to  colour  a  section.  Coloured  pencils  may  serve  for 
this  purpose,  but  in  a  note-book  such  colours  are  apt  to  get 
rubbed  and  smudged,  and  ordinary  water-colours,  therefore, 
are  preferable.  Those  who  have  an  artistic  aptitude  enjoy  a 
great  advantage,  and  can  often  do  without  the  help  of  square- 
ruling — bringing  out  with  a  few  deftly  drawn  lines  on  plain 
paper  all  the  geological  features  that  call  for  expression.  They 
can  fill  their  note-books  also  with  sketches  of  scenery  which 
may  show  at  a  glance  how  the  configuration  of  the  ground 
has  been  determined  by  the  nature  and  structure  of  the 
rocks.  If  the  observer  have  this  gift,  he  would  do  well  to 
cultivate  it — for  he  may  be  sure  that  his  descriptions  of 
geological  phenomena  will  gain  enormously  in  clearness  and 
value  when  they  are  accompanied  by  well-selected  artistic 
illustrations. 

To  others  who  have  not  been  blessed  with  artistic  talent, 
photography  lends  much  assistance,  and  is  therefore  largely 
indulged  in  by  field-observers — good  portable  cameras  being 
readily  obtainable. 

The  Topographical  Map. — Reliable  topographical  maps  of 
most  civilised  countries  can  now  be  obtained.  In  our  islands 
the  maps  issued  by  the  Ordnance  Survey  cannot  be  surpassed 
for  accuracy,  and  are  just  such  as  are  desiderated  by  the 
geologist.  These  maps  are  on  various  scales — those  on  the 
scale  of  six  inches  and  one  inch  to  the  mile  respectively  being 
most  useful  for  geological  purposes.  The  beginner  will  find 
the  larger  scale  map  the  more  satisfactory  of  the  two,  as  it 
enables  him  to  plot  his  observations  in  much  greater  detail 
than  would  be  possible  on  the  other.  The  shape  of  the 
ground  is  indicated  by  numerous  contour  lines  (i.e.  lines  of 
equal  elevation),  instead  of  by  "  hill-shading,"  so  that  pencilled 
notes  and  lines  are  clearly  seen,  and  the  observer  is  usuall 


278  STRUCTURAL  AND  FIELD  GEOLOGY 

saved  the  trouble  of  determining  heights  which,  for  various 
geological  purposes,  it  is  often  necessary  to  ascertain. 

When  large  maps  like  those  referred  to  are  not  available, 
and  the  observer  has  to  content  himself  with  maps  on  a  much 
smaller  scale,  he  may  occasionally  be  compelled  to  redraw 
portions  of  his  map  on  a  larger  scale.  Such  will  be  the  case 
when  the  geological  structures  are  so  highly  complicated  that 
they  cannot  be  indicated  save  in  a  generalised  way  on  a 
small  map.  Every  field  geologist's  note-book  is  sure  to 
contain  enlarged  sketch-maps  of  this  kind,  showing  in  detail 
complex  structures  which  it  would  be  impossible  to  represent 
upon  any  ordinary  topographical  map.  And  such  enlarged 
portions  of  his  map  may  serve  subsequently  as  illustrations  to 
accompany  the  observer's  monograph  or  paper  descriptive  of 
the  region  surveyed. 

The  maps  of  some  countries  which  are  only  sparsely 
settled  are  often  little  better  than  generalised  sketches, 
making  no  pretensions  to  accuracy ;  while  the  topography 
of  many  wide  regions  has  not  yet  been  delineated  even  in 
outline.  Geologists  in  such  cases  must  be  prepared  to  do 
some  topographical  surveying  for  themselves  if  they  wish  to 
prepare  a  geological  map.  In  several  of  our  colonies 
surveying  of  this  kind  has  been  carried  on  by  geologists 
concurrently  with  their  own  special  work.  Students  of 
geology,  therefore,  if  they  intend  emigrating,  should  certainly 
acquire  some  knowledge  of  topographical  surveying  before 
leaving  home.  Even  if  they  have  relatively  accurate  maps 
provided  for  them,  they  may  yet  frequently  find  it  necessary 
to  correct  these  or  to  lay  down  the  topography  in  greater 
detail. 

Geological  Data  to  be  Mapped. — Assuming  that  the 
student  begins  his  field  work  in  this  country,  he  has,  of 
course,  accurate  and  detailed  maps  at  his  service,  which  is 
a  very  great  advantage :  for  it  will  readily  be  understood 
that  when  the  topography  is  inaccurate  the  geological  lines 
cannot  be  otherwise  than  distorted.  An  approximately 
perfect  geological  map  must,  therefore,  in  the  first  place,  be 
thoroughly  accurate  as  regards  its  topography.  It  should 
also  be  on  not  too  small  a  scale,  for  the  larger  the  map  the 
greater  the  detail  that  can  be  shown,  and  the  more  readily 


GEOLOGICAL  SURVEYING  279 

and  exactly  are  geological  positions  determined.  To  be  of 
any  practical  use,  a  good  geological  map  ought  to  exhibit  the 
following  features,  viz.  : — 

(a)  The  superficial  areas  occupied  by  geological  systems 
and  their  chief  subdivisions — the  mutual  boundaries  of  the 
several  groups  or  series  being  accurately  delineated. 

(^)  Individual  seams,  beds,  or  formations  of  economic  or 
scientific  interest  and  importance,  such  as  coals,  limestones, 
ironstones,  etc. ;  the  position  of  available  building-stones, 
etc. ;  the  best  sources  of  underground  water-supply ;  the 
general  character  and  distribution  of  superficial  accumulations, 
subsoils,  and  soils. 

(c)  Igneous    rocks — effusive    being    clearly   distinguished 
from  intrusive  rocks. 

(d)  Faults,  and  all  fissures  which  may  be  supposed  likely 
to  contain  ore-formations. 

(e)  Dips  should  be  everywhere  carefully  inserted,  so  as  to 
show  exactly  the  direction  and   degree  of  inclination  of  the 
strata. 

A  map  containing  these  data  would  enable  a  geologist, 
who  might  never  have  visited  the  region  represented,  to 
understand  at  a  glance  the  geological  structure.  From  the 
details  given,  he  could  measure  the  thickness  of  the  strata, 
and  ascertain  the  depths  from  the  surface  at  which  particular 
seams  or  beds  might  be  expected  to  occur  at  given  points. 
He  would  be  in  a  position  to  indicate  where  an  underground 
water-supply  might  be  tapped  by  borings — all  this,  and  much 
more,  a  carefully  constructed  geological  map  will  reveal  to 
anyone  who  has  the  skill  to  read  it.  Only  large  scale  maps, 
such  as  those  issued  by  the  Geological  Survey  of  Great 
Britain,  are  sufficiently  detailed  to  be  used  in  this  way.  The 
field-observations  of  the  Survey  are  plotted  on  the  larger 
Ordnance  Map  (6  inches  to  a  mile),  and  the  sheets  repre- 
senting the  more  important  parts  of  the  country  are  published 
on  that  scale.  The  several  geological  systems  and  their  sub- 
divisions, and  the  general  structure  of  a  region,  however,  can 
be  quite  well  represented  on  a  smaller  scale.  The  Geological 
Survey,  for  example,  issues  a  general  map,  on  the  scale  of 
i  inch  to  a  mile,  the  information  given  on  which  is,  of  course, 
taken  from  the  larger  map.  Having  been  carefully  reduced 


280  STRUCTURAL  AND  FIELD  GEOLOGY 

from  the  6-inch  field-maps,  the  smaller  map  is  sufficiently 
accurate  and  detailed  for  general  purposes. 

The  maps  issued  by  national  geological  surveys  are 
seldom  on  a  larger  scale  than  I  inch  to  a  mile,  and  are 
usually  much  smaller.  Such  maps  do  little  more  than 
represent  the  broader  geological  features,  the  distribution 
of  the  several  systems  and  their  larger  subdivisions,  together 
with  the  more  important  developments  of  igneous  rocks, 
leading  lines  of  dislocation,  position  of  ore-deposits,  etc. 
They  are  accompanied,  however,  by  more  or  less  elaborate 
monographs,  which  contain  such  detailed  information  as 
could  not  be  expressed  upon  the  maps  themselves.  And 
the  geology  of  the  regions  represented  on  the  latter  is  still 
further  explained  by  means  of  horizontal  (or  profile)  and 
vertical  sections,  the  former  being  constructed  so  as  to  indicate 
or  represent  the  shape  of  the  surface  and  the  geological 
structure  of  the  ground,  while  the  latter  are  designed  to  show 
in  as  great  detail  as  possible  the  succession  of  important 
groups  or  series  of  strata,  such  as  coal-  or  ironstone-bearing 
formations.  (The  method  of  constructing  geological  sections 
is  set  forth  in  Chapter  XXI.) 

Small  generalised  geological  maps  on  a  scale  of  10  miles 
to  an  inch  or  less,  are  designed  to  show  merely  the  dis- 
tribution of  the  chief  rock-divisions,  and  have  usually  been 
reduced  from  larger  maps.  Sometimes,  however,  outline-  or 
sketch-maps  of  this  kind  are  original  productions,  accompany- 
ing the  descriptions  of  hitherto  unknown  or  imperfectly  known 
regions.  They  are  meant  to  do  no  more  than  illustrate  the 
pioneer  work  of  geological  explorers,  and  do  not  therefore 
make  any  pretension  to  minute  accuracy. 

To  the  student  who  would  become  an  expert  field- 
geologist,  topographical  maps  on  a  scale  of  less  than  I  inch 
to  a  mile  are  of  little  use.  Even  a  i-inch  map  cannot  be 
recommended  to  one  who  has  all  his  experience  to  gain. 
The  beginner  who  has  the  good  fortune  to  commence  work 
in  this  country  cannot  do  better  than  follow  the  example  of 
our  Geological  Survey  and  use  the  6-inch  Ordnance  Map. 
Although  this  map  is  large  enough  to  allow  here  and  there 
of  notes  being  inserted,  the  observer  will  soon  find  it  necessary 
to  use  abbreviations,  signs,  and  symbols.  For  example, 


PLATE  LI, 


SIGNS    USED    ON    MAPS    OF    H.M. 
GEOLOGICAL    SURVEY 


Signs  connected  with  the  Glacial  Drift 

CD         Roches  moutonnees  (not  striated). 

Roches   moutonnees    (striated),    but    direction    of  ice-flow    not 
apparent. 

Roches     moutonnees     (striated),    showing     direction    of    ice- 
flow. 

—0 —     Plane  or  flat  surface  (striated). 

Plane  or  flat  surface  (striated),  where  direction  of  ice-flow  is 
visible. 

Signs  connected  with  Stratification 


)\       Steeply  Inclined  Strata. 
r       Cleavage. 


Beds. 

— | —   Anticlinal  Axis. 


\      General  Dip  of  Undulating 


Horizontal 
I  Vertical 

(longest  line 
the  strike) 

j\f\j       Contorted    , 

Beds. 

Inclined  Strata.  ^     Undulating   Strata. 

^  Gently  Inclined  Strata.       — i — Synclinal  Axis. 

Interrupted  Lines _  show  a  doubtful  or  drift-covered  Boundary. 

White  Lines,  Faults. 

Signs  indicating  the  Ores  of  the  Metals 

O      Gold.  Y      Manganese.  Z      Tin. 


Lead. 


J>      Silver. 


Copper. 


cT      Iron. 


Zinc. 


Nickel. 


Gold  Lines,  Mineral  Veins. 

[To  face  page  280.. 


GEOLOGICAL  SURVEYING  281 

instead  of  writing  sandstones  and  shales,  SS  or  Sa  &  Sk  will 
suffice.  In  like  manner,  most  of  the  common  igneous  rocks 
can  be  indicated  by  means  of  the  initial  letters,  as  B  for 
basalt,  G  for  granite,  Sy  for  syenite,  D  for  dolerite,  Di  for 
diorite,  P  for  porphyrite,  and  so  on.  Plate  LI.  shows  some 
of  the  signs  and  symbols  used  by  the  Geological  Survey  of 
Great  Britain  and  Ireland. 

Tracing*  Exposed  Outcrops. — As  the  most  continuous 
exposures  of  rock  naturally  occur  upon  sea-coasts  and  along 
river-courses,  it  is  best  for  practice  to  select,  if  possible,  some 
tract  the  situation  and  topography  of  which  seem  to  promise 
the  observer  most  information.  Proceeding  along  the  sea- 
coast,  and  following  the  stream-courses  of  a  region  which  we 
shall  suppose  consists  largely  of  stratified  rocks,  the  student 
must  insert  upon  his  map  the  direction  and  angle  of  dip 
as  frequently  as  possible.  The  outcrops  of  all  notable  or 
important  beds  and  seams  (such  as  limestones,  coals,  iron- 
stones, etc.)  are  carefully  set  down,  and  particular  descriptions 
of  these  and  the  accompanying  strata  are  recorded  in  the 
note-book.  Fossils  are  sedulously  searched  for  everywhere, 
more  particularly  in  the  finer  grained  argillaceous  sandstones 
and  shales  amongst  which  seams  of  coal  and  ironstone  or 
beds  of  limestone  not  infrequently  occur.  Should  any  seam 
or  layer  be  characterised  by  the  presence  of  certain  fossils 
peculiar  to  itself,  the  exact  position  of  such  seam  should  be 
carefully  indicated,  for  it  may  be  of  great  service  as  a  datum- 
line  or  .geological  horizon,  as  will  be  shown  presently. 
Bedded  ironstones  and  limestones  are  often  marked  by  the 
presence  of  special  fossil-forms,  and  this  is  one  reason  why 
the  outcrops  of  such  rocks  are  invariably  mapped  by  a  field- 
geologist.  Any  stratum  or  series  of  strata,  however,  which 
may  be  notable  on  account  of  fossils  or  lithological  character, 
must  be  distinguished  from  immediately  overlying  and  under- 
lying strata.  Not  infrequently  it  is  possible  to  separate  a 
great  succession  of  sedimentary  deposits  into  subordinate 
groups — each,  it  may  be,  marked  by  the  presence  of  particular 
fossils,  or  by  the  composition  and  structure  of  the  rocks 
themselves. 

Tracing  Concealed  Outcrops. — After  the  observer  has 
examined  every  exposure  of  rock  upon  the  sea-coast,  in  river- 


282  STRUCTURAL  AND  FIELD  GEOLOGY 

courses,  and  elsewhere,  and  exhausted  all  the  evidence  to  be 
obtained  in  railway-cuttings,  quarries,  and  other  excavations, 
he  will  usually  find  that  there  are  wide  areas  over  which  no 
rock  appears  at  the  surface.  The  surface  may  be  concealed 
under  thick  soils  and  subsoils,  or  overspread  by  superficial 
accumulations  of  various  kinds,  as  clay,  sand,  gravel,  peat,  etc. 
How  are  such  blanks  in  the  evidence  to  be  filled  up  ?  How 
can  we  carry  the  lines  of  outcrops  across  tracts  which  are 
apparently  so  hopelessly  mantled  ?  Fortunately,  it  is  usually 
possible  to  follow  lines  of  outcrop  even  when  the  rocks  them- 
selves are  not  actually  seen,  for,  although  concealed,  their 
presence  is  often  betrayed  in  various  ways.  The  following 
are  some  of  the  sources  of  information  of  which  a  keen-eyed 
observer  will  avail  himself: — 

(a)  Soils  and  Subsoils. — In  regions  which  are  not  covered 
by  glacial  deposits  (such  as  boulder-clay),  or  by  thick  sheets 
of  transported  materials  (sand,  gravel,  etc.),  the  soils  will 
usually  indicate  the  nature  of  the  underlying  solid  rocks, 
fragments  of  which  are  almost  certain  to  occur  more  or  less 
abundantly.  These  will,  of  course,  be  readily  detected  in 
newly  ploughed  ground,  but  when  the  soil  is  carpeted  with 
vegetation,  information  is  nevertheless  often  obtainable  from 
worm-castings,  mole-heaps,  rabbit-burrows,  etc.  A  red,  sandy 
soil  containing  angular  fragments  of  red  sandstone  will  indicate 
the  presence  of  red  sandstone  underneath.  Tenaceous  clay- 
soils,  with  few  or  no  stones,  will  be  found  to  pass  downwards 
into  marls,  clays,  or  argillaceous  shales.  Should  subangular, 
blunted  stones  (some  of  them,  perhaps,  striated)  occur 
numerously  in  a  stiff  clay  soil,  the  presence  of  till  or  boulder- 
clay  is  indicated.  A  soil  charged  with  numerous  rounded 
water-worn  stones  will  be  found  to  overlie  either  a  superficial 
deposit  of  gravel  or  a  decomposed  conglomerate. 

In  estimating  the  value  of  the  evidence  furnished  by  surface 
stones,  it  is  well  to  remember  that  if  the  stones,  whether  sub- 
angular  or  rounded,  should  consist  of  many  different  kinds  of 
rock,  they  must  be  derived  from  an  underlying  superficial 
accumulation  of  transported  materials,  or,  as  just  remarked, 
they  may  indicate  the  presence  below  of  a  disintegrating 
conglomerate,  the  outcrop  of  which  the  observer  will  probably 
have  already  encountered  in  some  natural  or  artificial  ex- 


GEOLOGICAL  SURVEYING 


283 


posure — say,  in  sea-cliff,  river-course,  or  railway-cutting. 
Although  a  soil  be  charged  with  abundant  angular  fragments 
of  one  and  the  same  kind  of  rock,  it  does  not  necessarily 
follow  that  the  parent  rock  from  which  these  fragments  have 
been  derived,  will  be  encountered  immediately  underneath 
the  surface.  Much  will  depend  upon  the  configuration  or 
shape  of  the  ground.  All  soils  and  disintegrated  rock- 
materials  tend  to  travel  downwards  from  higher  to  lower 
levels,  and,  in  this  way,  soil  derived  from  one  kind  of  rock 
comes  to  overlap  and  to  be  commingled  with  soil  derived, 
it  may  be,  from  quite  a  different  class  of  rock-material.  The 
annexed  diagram  (see  Fig.  108)  will  serve  to  illustrate  this 
point.  Here  there  are  three  separate  beds  represented — a 


FIG.  1 08.— TRAVELLING  OF  SOIL  AND  SUBSOIL. 

being  a  dark  red  marl ;  £,  a  grey  sandstone ;  and  c,  a  coarse 
conglomerate.  The  soil  overlying  a,  which  occupies  the  top 
of  the  hill  or  bank,  is  red  and  argillaceous,  and  as  this  soil 
tends  to  travel  down  the  slope,  it  invades  the  outcrop  of  the 
stratum  b,  where  it  becomes  commingled  with  grey  sand, 
derived  from  the  disintegration  of  the  sandstone.  There  is 
thus  a  gradual  passage  from  a  pronounced  dark  red  clay  soil 
into  a  more  or  less  arenaceous  soil  of  a  lighter  tint — the  red 
colour  gradually  becoming  less  and  less  conspicuous  as  the 
base  of  the  slope  is  approached.  It  is  obvious  that  angular 
fragments  of  grey  sandstone  may  be  met  with  in  the  soil,  at 
all  levels  from  the  outcrop  of  b  downwards,  while  stones  from 
the  conglomerate  will  be  confined  to  the  soil  that  overlies  that 
stratum.  This  commingling  of  soil  and  disintegrated  rock-debris 
along  the  boundaries  of  formations  is  everywhere  observable, 


284  STRUCTURAL  AND  FIELD  GEOLOGY 

and  the  geologist,  therefore,  in  drawing  his  boundary-lines, 
must  make  the  necessary  allowances.  In  the  case  represented 
in  the  diagram  there  would  be  no  difficulty  in  ascertaining  the 
boundary  lines  between  the  several  beds.  Walking  up  the 
slope  the  presence  of  rounded  stones  would  indicate  the 
presence  of  the  conglomerate,  so  long  as  even  one  or  two  only 
appeared.  Above  the  junction  of  beds  c  and  b  water-worn 
stones  would  no  longer  be  met  with,  while  fragments  of  sand- 
stone might  continue  to  abound  until  the  limits  of  the  stratum 
b  were  reached.  The  position  of  the  boundary-lines  to  be 
drawn  would  thus  be  approximately  indicated. 

Although  the  colours  of  soils  are  invariably  due  to  the 
character  of  the  rocks  from  which  they  have  been  derived, 
the  observer  must  remember  that  the  colour  of  unweathered 
rocks  often  differs  greatly  from  that  of  their  disintegrated 
debris.  The  brown  and  reddish  colours  of  many  soils  are 
due  to  the  presence  of  iron-oxides,  but  such  soils  are  often 
derived  from  rocks  which  are  neither  brown  nor  red — these 
colours  having  resulted  from  the  chemical  alteration  of  the 
rocks.  Many  basic  igneous  rocks,  for  example,  which  may  be 
dark  blue  or  even  black,  yield  yellowish  and  reddish-brown 
soils.  Again,  some  kinds  of  blue  and  grey  boulder-clay  are 
overlaid  with  reddish-yellow  soils.  Many  impure  blue  and 
grey  limestones  also  tend  to  yield  yellowish  or  brownish  soils. 
Generally  speaking,  however,  the  colour  of  soils  formed  from 
the  disintegration  of  derivative  rocks  does  not,  for  obvious 
reasons,  differ  much  from  that  of  the  rocks  themselves. 
»"*  (b)  Character  of  Vegetation. — The  character  of  the  vegeta- 
tion is  often  an  index  to  the  nature  of  the  soil  and  underlying 
rocks  which  the  observer  cannot  always  afford  to  neglect.  It 
is  well  known  that  certain  plants  prefer  one  kind  of  soil  to 
another,  so  that  botanists  are  able  to  map  out  a  region  into 
areas  (not  always,  it  is  true,  sharply  defined),  each  of  which 
is  distinguished  by  the  development  of  some  particular 
assemblage  of  plants,  or  by  the  presence  of  certain  plants 
and  the  absence  of  others.  As  the  distribution  of  these 
plant-societies  depends  mostly  on  the  chemical  and  physical 
conditions  of  the  soil,  it  is  necessarily  suggestive  to  the 
geological  observer.  Soils  poor  in  carbonate  of  lime  show  a 
different  assemblage  of  plants  from  those  which  are  rich  in 


GEOLOGICAL  SURVEYING  285 

that    substance.     There    are    certain    species    (e.g.   common 
bracken,  common  heather,  sorrel,  fox-glove,  etc.)  which  avoid 
calcareous  soils ;  while,  on  the  other  hand,  not  a  few  species 
(e.g.  wild  cherry,  beech,  dogwood,  and  many  flowering  plants) 
are   particularly  partial   to   such   soils.     Porous   sandy  soils, 
tenacious  clay,  loose  loams,  saline  soils,  etc.,  are  each  char- 
acterised by  the  presence  of  distinctive  plant-groups.     In  the 
absence   of  rock-exposures,  therefore,  the   plant-associations 
referred  to  may  often  be  helpful  to  the   field-geologist,  and 
enable  him  to  draw   his   boundary-lines   with   more   or   less 
confidence.      He    must    bear    in    mind,   however,   that    the 
boundary-lines   suggested   by  the   varying   character   of  the 
vegetation   will  not  often  coincide  even  approximately  with 
the  junction-line   he    is    in  search   of.     Soils,  we  have  seen, 
tend  to  travel  down  slopes,  however  gentle  these  may  be,  and 
in  this  way  a  soil  rich  in  lime  may  eventually  come  to  overlie 
a  quartzose  sandstone  which  might  contain  hardly  a  trace  of 
lime ;  just  as,  on  the  other  hand,  a  barren,  infertile  sand  may 
in  time  invade  and  cover  rocks,  which,  if  left  exposed  to  the 
weather,  would  naturally  have  yielded  a  highly  fertile  soil. 

Nevertheless,  the  observer  who  has  a  sufficient  knowledge 
of  botany  will  not  infrequently  have  occasion  to  turn  this 
knowledge  to  good  account.  Having  ascertained  the 
character  of  the  flora  which  he  finds  growing  upon  soils  in 
places  where  their  derivation  from  the  underlying  rocks  can 
be  seen,  as  it  were,  taking  place,  the  appearance  of  a  like 
plant-assemblage  elsewhere  will  lead  him  to  suspect  the 
presence  of  the  same  rocks  below,  although  none  of  these 
may  be  actually  visible  at  the  surface. 

(c)  Form  of  Surface. — The  shape  or  configuration  of 
ground  is  frequently  of  great  service  in  showing  where  a 
boundary-line  should  be  drawn.  As  will  be  set  forth  more 
fully  in  the  sequel,  the  forms  assumed  by  a  land-surface  are 
determined  in  chief  measure  by  the  nature  of  the  underlying 
rocks  and  their  geological  structure.  Rocks  differ  greatly  as 
regards  durability — some  being  much  more  readily  reduced 
than  others  by  the  superficial  agents  of  change.  Hence,  in 
regions  which  have  been  long  exposed  to  denudation,  the  less 
readily  disintegrated  rocks  tend  to  project,  while  the  more 
yielding  kinds  are  correspondingly  worn-down  or  levelled. 


286 


STRUCTURAL  AND  FIELD  GEOLOGY 


It  is  matter  of  common  knowledge,  indeed,  that  hills  and 
ridges  are  usually,  or  at  least  very  often,  built  up  of  relatively 
harder  or  more  resistant  rocks  than  those  that  occupy  con- 
tiguous, low-lying  tracts.  This,  however,  is  not  invariably  the 
case,  as  will  be  shown  later  on.  Not  infrequently  the  hills  of 
a  country  consist  of  no  harder  or  less  readily  disintegrated 
rocks  than  are  found  in  the  low  grounds.  In  a  great  many 
cases  this  is  due  to  the  geological  structure  or  arrangement  of 
the  rocks.  There  are  certain  structures  that  tend  to  resist 


•  FIG.  109.— SURFACE-FEATURES  IN  GENTLY-FOLDED  SANDSTONES. 

while  others  favour  denudation.  Hence,  a  series  of  strata 
having  the  same  consistency  throughout,  may  in  some  places 
form  hills,  while  elsewhere  they  may  occupy  depressions  of 
the  surface.  In  the  above  diagram  (see  Fig.  109),  for 
example,  it  is  obvious  that  the  position  of  the  hills  has  been 
determined  by  the  strong  synclinal  arrangement,  while  the 
weaker  anticlinal  structures  have  been  more  readily  reduced. 
If  the  observer  be  geologising  in  a  region  where  the  rocks  are 
inclined  for  long  distances  in  the  same  direction,  he  will 
usually  find  that  the  outcrops  of  relatively  harder  beds  tend 


FIG.  no. — FORM  OF  GROUND  INFLUENCED  BY  GEOLOGICAL  STRUCTURE. 

to  project  more  than  those  of  the  less  durable  strata  amongst 
which  they  are  intercalated.  Hence,  even  when  the  naked 
rock  is  concealed  by  vegetation  and  soil,  it  nevertheless  will 
form  a  feature.  Thus,  in  the  accompanying  section  (Fig.  1 10), 
we  have  a  series  of  limestones  and  shales,  the  outcrops  of 
which  are  not  actually  seen,  and  yet  their  position  is  indicated 


GEOLOGICAL  SURVEYING  287 

by  the  form  of  the  ground.  It  is  obvious,  indeed,  that  the 
occurrence  of  a  thick  bed  of  relatively  hard  rock  intercalated 
in  a  series  of  softer  or  more  yielding  strata,  inclined  in  one 
and  the,  same  direction,  must,  under  the  influence  of  denuda- 
tion, give  rise  to  the  formation  of  escarpments  or  ridges — 
which,  whether  the  naked  rock  be  actually  exposed  or  not, 
will  form  prominent  features  in  a  landscape.  In  the  case  of 
countries  which  are  built  up  of  horizontal  strata,  the  varying 
hardness  of  the  rocks  will  similarly  affect  the  form  of  the 
ground,  and  cause  it  to  assume  a  terraced  aspect — a  structure 
illustrated  in  the  same  diagram  (Fig.  1 10),  where  the  gentler 
slopes  correspond  with  the  outcrops  of  "  soft "  rocks,  and  the 
more  abrupt  gradients  with  the  outcrops  of  "  hard  "  rocks. 

It  must  be  borne  in  mind,  however,  that  in  countries 
heavily  covered  with  glacial  and  other  superficial  accumula- 
tions, the  surface  configuration  of  the  underlying  solid  rocks 
is  often  obscured  or  even  entirely  concealed.  But  when  such 
deposits  are  either  absent  or  attain  no  great  thickness, 
the  form  of  the  ground  is  always  of  the  greatest  assistance  to 
the  geologist  who  is  trying  to  carry  a  line  of  outcrops  across  a 
country. 

(d)  Springs. — Considerable  aid  in  tracing  boundary-lines 
is  sometimes  afforded  by  springs.     When  layers  of  relatively 
impervious  materials,  such  as  shales,  clay,  etc.,  are  intercalated 
among  a  series  of  porous  strata,  underground  water  tends  to 
come  to  the  surface  along  the  line  of  junction  between  the 
porous  and  the  non-porous  strata.     This   will   often  happen 
when  bedded  rocks  are  truncated  by  the  slope  of  the  ground, 
the  water  appearing  in  the  form    of  springs   or   oozing   out 
slowly  and  giving  rise  to  marshy  and  damp  spots.     Should  a 
number  of  such  springs  or  seepage-places  occur  in  succession 
in   some   given   direction,  they  will   necessarily  indicate   the 
presence  of  a  geological  boundary-line.     Occasionally,  spring- 
water    is    highly    charged    with    mineral    matter,    such    as 
carbonate   of  lime,   iron-oxide,   etc.,  and   hence   deposits   of 
calc    sinter,   bog-iron   ore,   etc.,   tend   to   be   formed    at   the 
surface  along  the  junction  between  porous  and  impermeable 
strata.      (For   a    more    particular    account    of    springs,   see 
Chapter  XXIII.) 

(e)  Index-beds.— Although    it    is     true     that     the     most 


288  STRUCTURAL  AND  FIELD  GEOLOGY 

continuous  exposures  of  rock  are  to  be  met  with  along  the 
seashore  and  in  river-valleys,  it  nevertheless   often   happens 
that,  owing  to  the  presence  of  superficial  accumulations,  the 
rocks   in   a   valley  may  be   concealed   for   longer  or  shorter 
distances.     But  should  the  observer  have  previously  examined 
the  strata  over  a  considerable  area,  the  occurrence   of  such 
blanks  in  the  evidence  does  not  necessarily  disconcert  him. 
He  probably  recognises,  in  the  few  sections  available  for  study 
in  some  particular  valley,  portions  of  a  series   of  beds,  the 
stratigraphical  position  of  which  has  already  been  revealed  by 
more    continuous   sections   exposed   elsewhere   in   the   same 
district.     After  he  has  carefully  studied  the  strata  of  a  wide 
area,  he  will  frequently  find  that  a  great  thickness  of  strata 
may  show  a  monotonous  alternation  of  the   same   kinds   of 
rock,  say,  sandstones  and  shales,  and  yet  these  may  exhibit 
sufficient  variety  of  lithological  character  to  allow  of  the  whole 
series  being  roughly  divided.     Perhaps  thick-bedded  coarse- 
grained  sandstones   and   grits   with  subordinate  shales  may 
prevail  at  one  horizon,  and  shales  with  occasional  thin  beds  of 
fine-grained  sandstones  may  predominate  elsewhere.     Possibly, 
also,  the  shales  at  stated  intervals  may  contain  nodules  of  a 
particular  kind,  or  there  may  occur  at  a  definite  horizon  some 
stratum    characterised    either    by   its    fossils   or   by   certain 
peculiarities  of  composition,  texture,  or   structure.     Beds   of 
this  kind   are   not   infrequently  persistent   over   considerable 
areas,  and  when  such  is  the  case  they  are  invaluable  to  the 
field  geologist.     They  may  not  be  of  sufficient  importance  to 
be   mapped  separately  from  the  series  in  which  they  occur, 
but  their  presence  in  a  section  at  once  indicates  the  strati- 
graphical   horizon.      Should   the   observer   have    ascertained 
that  an  "  index-bed  "  of  this  nature  lies  at  a   given   distance 
above  or  below  any  limestone,  coal,  or  other  valuable  seam  he 
may  be  desirous  of  mapping,  it  is  obvious  that  the  appearance 
of  the  index-bed  in   a   valley  must   enable    him    to   fix   the 
approximate   position   of  the   seam   he    is    in   quest   of — no 
matter  how  deeply  the  outcrop  of  the  latter  may  be  buried 
under  alluvium.     The  field-geologist,  therefore,  cannot  be  too 
careful  in  acquiring  a  full  knowledge,  not  only  of  the  particular 
beds  whose  outcrops  he  seeks  to  trace,  but   of  the   varying 
characters  of  the  several  groups  or  series  of  strata  with  which 


GEOLOGICAL  SURVEYING  289 

those  beds  are  interstratified.  An  adequate  detailed  acquaint- 
ance with  the  whole  series  of  rocks  occurring  in  a  district 
often  enables  the  observer  to  locate  the  geological  horizon  of 
isolated  rock-exposures,  and  to  plot  the  position  of  boundary- 
lines  with  wonderful  accuracy,  even  in  places  where  the 
ground  is  thickly  mantled  with  superficial  deposits. 

(f)  Transported  Detritus  in  Stream-courses. — In  cases  where 
the  geological  position  of  the  rocks  exposed  in  a  stream  is  not 
suggested  by  the  character  of  the  rocks  themselves,  the  field- 
geologist  does  well  to  examine  carefully  the  gravel  and 
detritus,  as  he  proceeds  up  the  valley.  Should  he  detect 
fragments  of  a  rock,  say,  limestone,  which  he  has  already 
encountered  in  situ  elsewhere  in  the  same  district,  he  makes 
careful  note  of  his  find  and  continues  to  follow  the  spoor  up- 
stream. Possibly  the  limestone  fragments  become  more  and 
more  numerous  as  he  goes  on  his  way,  while,  at  the  same 
time,  they  are  less  water-worn,  and  occasionally,  perhaps 
attain  a  relatively  large  size.  Eventually,  at  some  particular 
spot  they  cease  to  occur — the  obvious  inference  from  which  is 
that  the  limestone  itself  must  crop  out  here  or  at  some  short 
distance  up-stream.  In  a  case  of  this  kind  a  geologist  would 
naturally  seek  to  strengthen  the  evidence  by  carefully 
examining  the  adjacent  valley-slopes  for  similar  angular 
fragments. 

After  direct  and  indirect  evidence  of  every  kind  has  been 
exhausted,  we  probably  find  that  there  are  still  certain  spaces 
upon  our  map  across  which  boundary-lines  cannot  be  traced. 
Wide  sheets  of  peat  or  alluvium,  for  example,  may  effectually 
conceal  broad  areas.  Should  the  map  we  are  using  be  on  a 
large  scale,  say  6  inches  to  the  mile,  we  should  stop  the  lines 
abruptly  where  they  meet  the  obscuring  sheet  of  alluvium  or 
peat,  and  colour  the  latter  as  a  separate  formation.  On  small- 
scale  maps,  however,  it  may  be  desirable  in  many  cases  to 
carry  a  line— more  especially  if  it  be  the  outcrop  of  some 
important  or  valuable  seam — across  areas  which  are  covered 
by  peat  or  alluvium.  This  may  be  safely  done  when  we  have 
assured  ourselves  that  there  is  no  interruption  or  break  in  the 
continuity  of  the  strata.  When  the  conditions  are  such  as 
represented  in  Fig.  in,  there  can  be  no  doubt  that  the  out- 
crop of  the  limestone  (a)  must  continue  across  the  area  con- 

T 


290 


STRUCTURAL  AND  FIELD  GEOLOGY 


cealed  by  the  peat  (.*•),  seeing  that  the  outcrop  of  the  upper 
limestone  (b)  has  been  followed  without  interruption  from 
west  to  east,  while  there  is  clear  evidence  of  a  continuous 
succession  of  strata  between  a  and  b. 

In  all  cases,  however,  where  an  outcrop  is  inferred  from 


FIG.  in. — CONCEALED  OUTCROPS. 

indirect  evidence,  a  conscientious  and  cautious  observer  will  be 
careful  to  indicate  this  by  drawing  dotted  or  interrupted 
instead  of  continuous  lines.  Continuous  lines  should  mean 
that  the  outcrops  are  actually  visible — that  the  rock  can  be 
seen  in  situ  ;  while  interrupted  lines  should  merely  indicate 
the  position  at  or  near  which  the  observer  thinks  it  likely 
that  the  outcrop  may  be  found. 


CHAPTER   XIX 
GEOLOGICAL  SURVEYING — continued 

Forms  of  Outcrop.  Measurement  of  Thickness  of  Strata.  Thickening 
and  Thinning  of  Strata.  Unconformity.  Overlap.  Normal  Faults. 
Reversed  Faults.  Eruptive  Rocks  and  Contact  Metamorphism. 
Regional  Metamorphism.  Archaean  Gneissose  Rocks. 

Forms  of  Outcrop. — The  form  and  direction  of  an  out- 
crop naturally  vary  with  the  configuration  of  the  ground  and 
the  direction  and  angle  of  dip.  As  a  rule,  the  most  winding 
and  sinuous  outcrops  appear  among  horizontal  strata,  especially 
when  these  have  been  deeply  trenched  and  eroded.  Gently 
inclined  strata  also  frequently  yield  very  sinuous  outcrops, 
while  the  outcrops  of  steeply  inclined  and  vertical  beds  are 
usually  more  regular  in  their  trend,  and  sometimes  run  for 
long  distances  in  approximately  straight  lines. 

Horizontal  Strata. — In  the  case  of  an  undulating  plateau 
built  up  of  horizontal  strata,  and  traversed  in  different  direc- 
tions by  many  valleys,  the  outcrops  necessarily  follow  all  the 
windings  of  the  latter — they  play  the  part,  in  short,  of  contour- 
lines.  The  width  of  the  outcrops  is  determined,  of  course, 
by  their  position  with  regard  to  the  configuration.  Thus, 
upon  a  steep  slope,  an  outcrop  of  a  stratum  many  feet  or 
yards  in  thickness  will  be  indicated  upon  the  map  by  a 
relatively  narrow  banol  or  ribbon,  while  the  outcrop  of  the 
same  stratum  occurring  on  the  top  of  a  hill  would  be  repre- 
sented by  the  whole  surface  of  the  bed,  which  might  form 
quite  a  broad  patch  of  colour  on  the  map. 

Inclined  Strata. — The  outcrops  of  inclined  strata  also  vary 
in  direction  with  the  shape  of  the  ground,  but  they  are 
influenced  likewise  by  the  angle  of  dip — an  influence  which 
becomes  less  and  less  marked  as  the  dip  increases.  The 

291 


292 


STRUCTURAL  AND  FIELD  GEOLOGY 


width  of  individual  outcrops  similarly  varies  with  the  degree 
of  inclination :  beds  dipping  at  a  low  angle  yielding  a 
relatively  broad  outcrop,  while  the  same  beds  dipping  at  a 
high  angle  present  a  relatively  narrow  outcrop.  Thus  the 
outcrop  of  a  bed  of  uniform  thickness  will  appear  broader  or 
narrower  as  the  dip  diminishes  or  increases. 

Vertical  Strata. — The  outcrops  of  vertical  beds  are  practi- 
cally uninfluenced  by  the  form  of  the  ground,  and  display,  of 
course,  the  true  thickness  of  the  strata. 

Measurement  of  Thickness  of  Strata. — When  strata  are 
horizontal,   it   is   obvious   that   their  thickness   can  only  be 


FIG.  112. — MEASUREMENT  OF  INCLINED  STRATA. 

measured  when  they  are  exposed  in  section,  as  in  sea-cliffs, 
river- valleys,  etc.  If  we  know  the  heights  above  sea-level 
reached  respectively  by  the  lowest  and  uppermost  beds  of 
a  great  series  of  horizontal  strata,  we  of  course  I  know  at  the 
same  time  the  thickness  of  the  strata.  So,  again,  in  the  case 
of  vertical  strata  it  is  obvious  that  a  line  measured  exactly 
across  the  strike  of  the  beds  will  give  us  their  thickness 
between  any  two  selected  points.  But  when  strata  are 
inclined,  the  width  of  their  outcrop  is  necessarily  greater 
than  the  actual  thickness  of  the  beds.  By  means  of  a 
protractor,  however,  there  is  no  difficulty  in  measuring  the 
thickness  of  a  series  of  strata,  inclined  at  a  known  angle 
between  any  two  given  points.  Thus,  in  the  diagram  (Fig. 


GEOLOGICAL  SURVEYING  293 

1 1 2),  we  have  a  series  of  beds  dipping  from  A  to  B  at  an 
angle  of  45°.  The  section  is  on  the  scale  of  6  inches  to 
the  mile,  so  that  the  width  of  the  outcrop  between  A  and 
B  is  880  yards,  or  half  a  mile.  All  that  we  need  to  do,  then, 
is  by  means  of  a  protractor  to  draw  lines  in  order  to  show 
the  exact  inclination  of  the  beds  at  A  and  B  respectively ; 
thereafter,  another  line  drawn  at  right  angles  to  the  dip 
from  a  to  b  gives  the  thickness  of  the  series  (640  yards). 
From  A  to  B  the  beds  dip  continuously  at  the  same  angle, 
but  this  is  not  very  often  the  case ;  more  commonly  the 
dip  is  apt  to  vary  in  amount  from  place  to  place.  When 
this  is  so,  all  we  can  do  is  to  take  the  average  angle  of 
inclination,  and  from  that  we  get  approximately  the  true 
thickness. 

The  following  rule,  given  by  Charles  Maclaren  in  his 
well-known  Geology  of  Fife  and  the  Lothians,  may  be  found 
serviceable  in  estimating  thicknesses  in  the  field.  If  the 
breadth  of  inclined  strata  be  measured  across  their  outcrop 
at  right  angles  to  the  strike,  their  true  thickness  will  be 
equal  to  TVth  of  their  apparent  thickness  for  every  5°  of 
inclination.  Or  the  rule  may  be  put  thus :  divide  60  by 
the  dip,  and  you  obtain  the  fraction  which  expresses  the 
thickness.  Thus,  suppose  a  series  of  strata  measures  across 
the  strike  1200  feet — if  the  dip  of  the  beds  be  5°,  their 
thickness  is  TM:h,  or  100  feet ;  if  the  dip  be  10°,  the  thickness 
is  Jth,  or  200  feet;  with  a  dip  of  15°  we  get  a  thickness  of 
^th,  or  300  feet ;  and  a  thickness  of  Jrd  or  400  feet,  when  the 
dip  is  20°.  The  rule  is  not  quite  correct  when  the  dip  exceeds 

45°. 

Thickening  and  Thinning  of  Strata. — When  the  observer 
has  completed  the  drawing  of  his  boundary-lines  and  out- 
crops, and  clearly  established  the  true  succession  of  the 
strata,  he  will  often  find  that  the  interval  between  the  out- 
crops of  two  separate  seams  or  beds  varies  from  point  to 
point.  In  other  words,  the  intermediate  strata  seem  to 
thicken  out  or  to  thin  away,  according  as  the  outcrops  are 
followed  in  one  direction  or  another.  Now  this  apparent 
increase  or  decrease  may  sometimes  be  accounted  for,  as 
we  have  seen,  by  inequalities  of  the  surface,  or  by  variations 
in  the  angle  of  the  dip.  If  we  have  satisfied  ourselves, 


294 


STRUCTURAL  AND  FIELD  GEOLOGY 


however,  that  the  mutual  approach  or  retreat  of  the  out- 
crops is  not  due  either  to  the  form  of  the  ground  or  to 
increase  or  decrease  in  the  degree  of  inclination,  then  we 
may  conclude  that  it  is  owing  to  an  actual  thinning-away 
or  thickening-out  of  the  intermediate  strata. 

Unconformity. — This  structure  is  readily  revealed  by 
mapping  the  ground,  even  although  it  may  never  be  shown 
in  any  actual  section.  The  accompanying  diagram  (Fig. 
113)  represents  the  ground-plan  of  an  unconformity.  Here 
there  are  two  scries  of  strata  inclined  in  different  directions 
— one  set  is  said  to  "  strike  at  or  against "  the  other.  It  is 
obvious  that  the  series  A  cannot  possibly  belong  to  the 


FIG.  113. — GROUND-PLAN  OF  AN  UNCONFORMITY. 

Continuous  lines  =  outcrops  and  boundaries  exposed  in  section.    Interrupted  lines  =  inferred 
positions  of  outcrops  and  boundaries.    Stippling = rocks  exposed  at  surface. 

series  B.  There  is  no  room,  so  to  say,  for  the  beds  A  to 
swing  round  (between  a  and  x)  and  dip  underneath  B. 
The  junction  between  the  two  series  must,  therefore,  be 
discordant,  and,  if  not  due  to  faulting,  can  only  indicate 
an  unconformity.  If  the  observer  have  reason  to  suspect 
an  unconformity,  he  must  carefully  look  for  such  evidence 
as  is  referred  to  in  Chapter  XII. — where  the  phenomena  of 
unconformity  and  overlap  are  described. 

Overlap  is  not  readily  shown  upon  a  plan  except  when  it 
accompanies  well-marked  unconformity.  Mapping  almost 
invariably  discloses  the  structure,  however,  when  it  occurs  on 
a  considerable  scale.  Small  local  overlaps  may  readily  be 
overlooked,  but  when  the  structure  characterises  a  wide  area 


GEOLOGICAL  SURVEYING 


295 


it  can  hardly  be  missed.  In  Fig.  1 14,  which  is  a  ground-plan 
of  an  unconformity  and  overlap,  the  latter  structure  is  not 
shown  in  the  area  traversed  by  the  section  line  A — B.  Here 
no  appearance  of  overlap  is  apparent,  but  as  the  outcrops  are 
followed  towards  the  area  C  D,  the  bed  b  gradually  overlaps 
the  bed  a,  while  the  former  is  overlapped  by  the  sandstone  c 
which  come  to  rest  directly,  and  with  a  strong  unconformity, 
on  the  highly  inclined  strata  S  S.  The  section  C  D  shows 
the  overlap  and  unconformity — b  overlapping  a,  and  being  in 
turn  overlapped  by  c. 


FIG.  114. — GROUND-PLAN  OF  UNCONFORMITY  AND  OVERLAP. 

S  S  =  Silurian  strata ;  x  x.  unconformity  ;  a,  b,  c,=younger  series  of  strata  showing 
overlap  ;  l.iSection  along  the  line  A — B  ;  2.  Section  along  the  line  C— D. 

Normal  Faults. — Faults  are  not  infrequently  observed  in 
natural  sections,  but  these,  as  a  rule,  are  small  downthrows  of 
little  importance.  The  larger  dislocations  of  a  faulted  region 
may  now  and  again  be  encountered  in  railway  cuttings  and 
other  excavations,  but  they  are  rarely  observed  in  natural 
rock  exposures.  One  reason  for  this  is  obvious  enough : 
highly  shattered  rocks  are  usually  associated  with  great 
faults,  so  that  when  these  are  exposed  by  denudation  the 
shattered  materials  readily  fall  asunder  and  the  actual  fracture 
becomes  concealed.  Or  the  shattered  rocks  on  one  or  both 
sides  of  the  fault  being  easily  broken  up  and  removed  by 
epigene  action,  a  hollow  may  be  washed  out  along  the  line 
of  dislocation,  and  form  eventually  a  receptacle  for  alluvium 
and  other  products  of  surface  erosion.  Many  faults  fail  to 
show  at  the  surface  of  regions  which,  like  our  own,  have  long 
been  exposed  to  denudation,  simply  owing  to  the  fact  that  any 
inequalities  of  surface  which  may  originally  have  been  caused 


296  STRUCTURAL  AND  FIELD  GEOLOGY 

by  such  dislocations  have  long  ago  been  planed  away,  and  the 
ground  has  become  more  or  less  swathed  in  soils,  subsoils,  and 
superficial  accumulations  of  every  kind.  Although  denudation 
tends  in  this  way  to  reduce  a  land-surface  generally,  neverthe- 
less it  is  obvious  that  hard  rocks  will  not  be  so  readily 
reduced  as  soft  rocks.  Thus  any  marked  or  sudden  change 
in  the  form  of  the  ground  will  be  due  in  most  cases  to  an 
abrupt  change  in  the  petrographical  character  or  the  geological 
structure  of  the  rocks.  In  most  cases  the  low  grounds  will 
be  composed  of  weakly  arranged  or  of  relatively  soft  rocks, 
while  the  higher  ground  will  indicate  the  presence  of  harder 
rocks,  or  stronger  structures  better  fitted  to  withstand  the 
destructive  action  of  epigene  agents.  As  one  frequent  result 
of  great  faults  is  to  bring  relatively  soft  and  hard  rocks 
together,  we  may  expect  to  find  that  such  faults,  although 
not  actually  seen  in  section,  have  yet  influenced  the  form 
assumed  by  the  ground  under  the  influence  of  denudation — 
the  hard  rocks  will  tend  to  project  above  the  level  of  the 
relatively  soft  or  less  durable  rocks. 

Abrupt  changes  in  the  form  of  the  ground  may  be  due,  however,  to  other 
structures  than  faults— the  more  important  of  which  are  the  following  :— 

(a)  An  abrupt  change  of  level  may  be  caused  by  the  outcrop  of  a 
relatively  durable  stratum  occurring  in  a  series  of  less  durable  strata.  In 
the  annexed  diagram  (Fig.  115)  we  have  the  structure  known  as  escarp- 


FIG.  115. — ESCARPMENT  AND  DIP-SLOPE. 

1),  dolerite ;  s,  sandstones  and  shales. 

ment  and  dip-slope—one  of  the  commonest  forms  of  land-surface, 
especially  in  regions  of  moderately  inclined  strata.  Now  and  again  the 
dip  of  the  strata,  instead  of  being  towards  the  high  ground  as  in 
escarpment-structure,  may  be  in  the  opposite  direction.  This  occurs 
when  a  thick  series  of  relatively  hard  rocks  are  overlaid  by  softer  strata. 
The  former,  as  in  other  cases,  tend  to  form  a  line  of  heights,  but  the 
descent  from  these  to  the  low  grounds  is  usually  less  abrupt  than  in  the 
case  of  escarpments  (see  Fig.  116).  In  both  cases  the  lines  of  elevation 
caused  by  such  outcrops  may  be  either  very  sinuous  or  approximately 


GEOLOGICAL  SURVEYING  297 

straight— according  as  the  strata  are  gently  or  steeply  inclined.  If  an 
escarpment  be  due  to  the  outcrop  of  such  a  rock  as  limestone  it  will 
usually  extend  for  some  considerable  distance.  If,  on  the  other  hand,  it 
has  been  determined  by  the  presence  of  a  sill  or  a  thick  conglomerate, 
its  lateral  extension  will  probably  be  limited. 

(£)  A  sudden  change  in  the  form  of  the  ground  may  indicate  an 
unconformity  (see  Fig.  113),  where  a  series  of  "soft"  rocks  (B)  repose 
on  the  truncated  ends  of  much  older  and  more  indurated  strata  (A). 
In  a  case  of  this  kind  the  line  of  high  ground  will  usually  be  more  or 
less  sinuous  and  irregular,  for  it  simply  represents  a  former  coast-line, 


FIG.  116.  —  INCLINED  "SOFT"  ROCKS  OVERLYING  "HARD"  ROCKS. 

the  younger  rocks  ever  and  anon  extending  into  what  were  old  bays 
and  inlets.  Evidence  of  so  well-marked  an  unconformity  as  this  could 
hardly  escape  an  observer  ;  but  in  nature  the  proofs  of  unconformity 
are  not  always  so  conspicuous. 

The  observer  who  encounters  a  sudden  or  abrupt  change 
from  low  to  high  ground,  and  has  satisfied  himself  that  the 
form  of  the  surface  cannot  be  explained  either  by  the 
occurrence  of  interbedded  hard  rocks,  of  intrusive  rock,  or 
of  an  unconformity,  will  be  justified  in  suspecting  the  presence 
of  a  fault.  If  a  fault  be  present,  then  the  line  separating 
low  and  high  ground  will  be  somewhat  straight  or  very 
gently  sinuous,  while  seepage  of  water  and  more  or  less 
numerous  springs  will  probably  occur,  and  so  indicate  the 
position  of  the  actual  line  of  fracture.  When  the  presence 
of  a  fault  is  thus  suspected,  the  field-geologist  will  carefully 
search  for  the  more  direct  evidence,  some  account  of  which 
has  been  given  in  Chapter  XI.  The  fault  itself,  if  it  be 
one  of  considerable  displacement,  will  probably  not  appear 
in  section,  but  he  may  find  that  the  strata  seen  in  the  low 
ground  become  more  or  less  abruptly  turned  up  at  high  angles 
of  inclination  as  he  approaches  the  base  of  the  hilly  ground, 
until,  at  last,  they  may  stand  on  end.  Should  such  be  the 
case,  the  strata  so  disturbed  will  probably  be  abundantly 


298 


STRUCTURAL  AND  FIELD  GEOLOGY 


FIG.  117.— FAULTED  STRATA  STRIKING 
AT  EACH  OTHER. 


shattered  and  traversed  in  all  directions  by  irregular  joints, 
the  faces  of  which  will  frequently  show  slickensides  and  be 
coated  with  mineral  matter.  In  short,  veins  of  quartz,  calcitc, 
etc.,  may  ramify  more  or  less  abundantly  through  the  disturbed 
rock-masses.  When  the  strata  on  both  sides  of  the  inferred 
fault  are  mapped,  the  observer  will  most  likely  find  that  the 
two  sets  of  rock  "  strike  at "  each  other — the  outcrops  of  one, 
or  it  may  be  of  both  series,  being  truncated,  as  shown  in  the 
accompanying  ground-plan  (Fig.  117).  The  determination 
of  the  downthrown  side  of  a  very  large  fault  is  seldom 

doubtful.  If  the  rela- 
tive age  of  the  strata 
on  either  side  of  the 
dislocation  be  known, 
and  this  is  usually  the 
case,  the  younger  rocks 
will,  of  course,  occur  on 
the  downthrow  side.  In 
cases  where  faults  tra- 
verse one  and  the  same 
series  of  rocks,  and  are 

not  exposed  anywhere  in  section,  the  downthrow  side  of  a 
dislocation  will  yet  be  rendered  evident  by  the  effect  produced 
upon  the  outcrops,  as  already  described  (Chapter  XL). 

Reversed  Faults. — Reversed  faults  occurring  amongst 
strata  the  geological  age  of  which  is  known  are  not  hard 
to  detect.  When  Carboniferous  rocks,  for  example,  are  seen 
dipping  regularly  underneath  Devonian  beds,  it  is  obvious 
that  this  inversion  of  the  stratigraphical  succession  must  be 
due  either  to  overfolcling,  or  to  an  overthrust,  or  to  a  com- 
bination of  both  structures.  If  the  inversion  be  the  result 
of  folding  alone,  then  it  is  obvious  that  both  series  of  rocks 
occurring  in  the  reversed  limb  of  a  strongly  unsymmetrical 
or  recumbent  fold  must  be  turned  upside  down.  If,  on  the 
other  hand,  the  inversion  has  been  caused  by  a  reversed 
fault  alone,  then  there  will  be  no  overturning  of  the  beds  in 
either  series  of  strata,  the  individual  beds  of  the  Carboniferous 
will  occur  in  regular  sequence,  and  the  same  will  be  the  case 
with  the  Devonian  strata.  But,  as  reversed  faults  have  fre- 
quently resulted  from  the  yielding  of  unsymmetrical  folds,  it 


GEOLOGICAL  SURVEYING  299 

often  happens  in  cases  of  inversion  that  this  structure  shows 
a  combination  of  overturning  and  overthrusting. 

Folding  and  faulting  of  such  extreme  kinds  are  usually 
best  developed  in  regions  which  have  been  subject  to  great 
deformation — regions  the  structural  geology  of  which  can 
hardly  be  unravelled  by  the  tyro.  The  observer,  who  is 
prepared  to  work  out  complicated  structures  like  those  of 
N.W.  Scotland,  the  Alps,  etc.,  has  got  far  beyond  the  need 
of  an  elementary  hand-book.  The  beginner,  however,  who 
is  anxious  to  become  familiarised  with  the  phenomena  likely 
to  be  encountered  in  regions  of  complex  structure,  can  hardly 
do  better  than  study  the  beautiful  maps  of  Wester  Sutherland 
and  Ross,  which  have  been  issued  by  the  Geological  Survey. 
With  these  maps  before  him,  and  with  the  help  of  the  works 
mentioned  in  the  footnote,  the  student  will  have  some  idea 
of  the  nature  of  rock-structures  which  are  characteristic  of 
"  folded  mountains."  * 

Eruptive  Rocks. — The  mapping  of  eruptive  rocks  is 
carried  on  in  the  same  way  as  that  of  sedimentary  strata. 
The  outcrops  of  contemporaneous  or  effusive  igneous  rocks 
are  not  more  difficult  to  follow  than  those  of  limestone  or  any 
other  bedded  rock.  The  boundary-lines  of  intrusive  bosses, 
sills,  and  dykes,  however,  are  more  irregular,  and,  in  the 
absence  of  sections,  may  sometimes  be  hard  to  trace.  Rocks 
of  this  class,  however,  are  usually  more  resistant  than  the 
rocks  they  traverse,  and  thus  tend  to  project  and  form  con- 
spicuous features  at  the  surface.  In  mountainous  regions 
where  the  rocks  are  generally  well  exposed,  the  field-geologist 
is  more  likely  to  be  troubled  with  the  abundance  than  with 
the  paucity  of  the  evidence.  In  the  case  of  a  mass  of  granite, 
for  example,  the  junction-line  is  apt  to  be  very  irregular — 
larger  and  smaller  veins  penetrating  the  adjacent  rocks  in  all 
directions.  The  details  of  structure  are  often,  indeed,  so 
intricate  that  the  most  the  observer  can  do  is  to  generalise 
*  The  sheets  of  the  i-inch  map  are  as  follows  : — 81,  91,  101,  107,  114. 
Consult  Annual  Reports  of  the  Geological  Survey,  1892-96  inclusive  ; 
Summary  of  Progress  of  H.M.  Geological  Survey  for  1897.  See  also 
Quarterly  Journal  of  the  Geological  Society,  vols.  xliv.  378  ;  xlviii.  227  ; 
1.  66 1.  The  whole  region  is  described  in  great  detail  in  the  Geological 
Survey's  Memoir — The  Geological  Structure  of  the  North-West  High- 
lands of  Scotland,  1907, 


300  STRUCTURAL  AND  FIELD  GEOLOGY 

these,  drawing  his  lines  so  as  to  show  the  shape  of  the  mass — 
whether  it  be  circular,  elliptical,  or  quite  irregular,  or  following 
in  a  rude  way  the  strike  of  the  surrounding  rocks.  The 
numerous  veins,  etc.,  must  be  generalised,  but  when  well 
exposed  in  section  or  in  plan,  it  is  advisable  to  make  careful 
drawings  of  these  for  future  reference,  when  the  phenomena 
come  to  be  described.  So  far  as  he  can  do  so,  the  observer 
will  try  to  indicate  upon  his  map  the  nature  of  the  altered 
rocks  which  surround  the  granite.  The  stages  of  contact- 
metamorphism,  however,  so  frequently  graduate  into  each 
other,  that  it  is  often  quite  impossible  to  draw  boundary-lines 
separating  one  kind  of  metamorphic  rock  from  another. 
Nevertheless  this  can  sometimes  be  done,  more  especially  in 
cases  where  the  original  unaltered  rocks  have  differed 
markedly  in  character,  and  have  thus  been  metamorphosed 
into  more  or  less  strongly  contrasted  sub-crystalline  and 
crystalline  rocks.  There  are  many  other  observations  that 
the  field-geologist  will  find  it  impossible  to  indicate  upon  a 
map,  but  which  he  should  not  fail  to  describe  in  his  note- 
book. 

Sills  are  not,  as  a  rule,  hard  to  trace.  Even  when  the 
actual  lines  of  junction  with  adjacent  rocks  are  not  exposed, 
the  intrusive  character  of  a  sill  is  frequently  indicated  by 
the  way  in  which  it  seems  to  steal  across  the  strike  of  the 
strata.  The  absence  of  any  bedded  tuff  accompanying  the 
igneous  rock,  would  be  so  far  suggestive  of  the  intrusive 
character  of  the  latter.  This  negative  evidence,  however, 
would  be  much  strengthened  if  veins  of  the  same  kind  of 
rock  were  found  penetrating  the  overlying  strata.  We  could 
hardly  doubt  in  that  case  that  the  veins  were  genetically 
connected  with  the  igneous  rock,  and  that  the  latter  therefore 
was  not  truly  bedded,  but  an  intrusive  sheet  or  sill.  Dykes 
are  even  more  readily  diagnosed  in  the  field  than  sills,  and 
can  usually  be  followed  without  difficulty.  Their  presence  is 
often  revealed  by  lines  of  springs  which  come  to  the  surface 
on  that  side  of  a  dyke  towards  which  the  strata  are  inclined. 
For  the  various  details  of  structure  and  the  general  pheno- 
mena characteristic  of  sills  and  dykes,  however,  reference 
should  be  made  to  Chapters  XIII.  and  XIV. 

The  same  chapters  also  give  some  account  of  Necks  or 


GEOLOGICAL  SURVEYING  301 

pipes  of  eruption.  When  these  structures  are  seen  either  in 
plan  or  in  section,  their  character  is  at  once  revealed.  Some- 
times, however,  the  actual  line  of  junction  between  them  and 
the  rocks  they  traverse  is  entirely  concealed,  and  in  such 
cases  they  might  possibly  be  mistaken  for  outliers.  Fig.  118, 
for  example,  shows  in  ground-plan  field  data  which  are  so 
apparently  incomplete  that  the  agglomerate  and  tuff  a,  might 
be  explained  as  an  outlier  resting  unconformably  upon  the 
truncated  ends  of  the  strata  b.  We  should  have  no  doubt, 
however,  as  to  its  intrusive  nature  if  we  could  make  the 
following  observations: — i.  The  tuff  either  shows  no  bedded 
arrangement,  or,  if  any  trace  of  bedding  be  visible,  the  dip  of 
the  rude  layers  is  towards  the  centre  of  the  mass ;  2.  Dykes 
or  veins  of  crystalline  rock  traverse  the  tuff,  while  similar 
veins  of  the  same  rock  appear  at  some  little  distance  invading 


FIG.  1 1 8. — GROUND-PLAN  OF  NECK. 

Continuous  lines  and  arrows  —  boundaries  and  dips  exposed  in  section.    Interrupted  lines ; 
conjectural  positions  of  boundaries.    Stippling,  etc.  =  rocks  exposed  at  surface. 


the  adjacent  rocks ;  3.  The  surrounding  strata  as  they 
approach  the  tuff  are  more  or  less  shattered,  and  perhaps 
show  traces  of  induration  as  if  from  the  action  of  heat. 
Should  the  portions  exposed  be  not  far  from  the  concealed 
junction,  the  beds  may  appear  suddenly  to  bend  over  so  as  to 
dip  abruptly  towards  the  agglomerate  or  tuff;  4.  Fragments 
of  the  adjoining  rocks  and  of  rocks  which  may  be  recognised 
as  belonging  to  lower  and  higher  geological  horizons,  can  be 
detected  in  the  tuff.  Evidence  of  this  varied  kind  would 
satisfy  us  that  the  igneous  rock  was  not  an  outlier  but  a 
neck,  and  we  should  be  justified  in  drawing  around  it  an 
interrupted  line. 


302 


STRUCTURAL  AND  FIELD  GEOLOGY 


Occasionally,  necks  are  occupied  wholly  by  crystalline 
rock,  the  junction  between  which  and  the  adjacent  rocks  may 
similarly  be  concealed,  so  that  the  observer  may  be  in  doubt 
at  first  as  to  whether  the  igneous  rock  may  not  be  an  isolated 
patch  or  cake  resting  unconformably  on  the  strata  that  crop 
out  in  its  immediate  neighbourhood.  Were  such  its  origin, 
its  jointing  should  be  vertical.  On  the  other  hand,  if  it  be  of 
the  nature  of  a  plug  occupying  a  pipe  of  eruption,  the  joint- 
planes  will  be  arranged  horizontally.  A  geologist  having 
satisfied  himself  on  this  point  would,  of  course,  seek  to 
strengthen  the  evidence  by  such  additional  observations  as 
are  referred  to  above  in  connection  with  necks  of  agglomerate. 

Slaty  Cleavage. — This  structure,  we  have  seen,  occurs  among  rocks 
which  have  been  more  or  less  folded  and  compressed.  In  fine-grained 
slates  the  original  planes  of  lamination  and  bedding  are  usually  obscured, 
and  may  even  be  entirely  obliterated,  and  when  such  is  the  case  the 
superinduced  cleavage-structure  might  readily  be  mistaken  for  planes  of 
sedimentation.  The  geologist,  therefore,  must  be  on  his  guard,  and 
when  any  thick  belt  of  finely  divided  argillaceous  rock  is  encountered  in 
a  region  of  steeply  inclined  and  much-folded  strata,  he  should  at  once 


FIG.  119.— CLEAVAGE  AND  BEDDING. 

suspect  that  the  division-planes  maybe  those  of  cleavage.  If  the  rock 
be  really  a  slate,  careful  examination  will  probably  result  in  the  detection 
of  the  original  lines  of  bedding.  These  may  be  indicated  by  alternating 
bands  of  differently  tinted  slate  or  by  variations  in  the  texture  of  the 
slates — such  differences  of  colour  and  texture  being  visible  in  section,  as 
it  were,  on  the  cleavage-planes.  By  splitting  the  slate  open  we  can  see 
that  the  varying  tint  and  texture  are  not  merely  superficial  but,  penetrating 
the  rock,  are  as  conspicuous  on  one  face  of  the  slate  as  on  the  other. 
Usually,  however,  bands  and  beds  of  greywacke,  quartzite,  or  other  less 
cleavable  rock  occur  interbedded  with  slates — and  the  presence  of  these 
at  once  discloses  the  true  bedding.  It  is  not  uncommon,  moreover,  to 
find  the  cleavage-structure  restricted  to  the  argillaceous  rocks  of  a  series 
of  folded  strata,  and  as  the  structure  in  question  frequently  traverses  the 
original  bedding-planes  at  a  high  angle,  the  junction  between  cleaved 
and  non-cleaved  rocks  often  resembles  an  unconformity  (Fig.  119).  In 


GEOLOGICAL  SURVEYING  303 

mapping  slates,  therefore,  the  chief  danger  to  be  avoided  is  the  mistaking 
of  cleavage  for  bedding.  It  is  necessary,  however,  always  to  note  the 
direction  of  the  strike  and  dip  of  the  cleavage-planes,  especially  when 
the  bedding  is  obscure  or  obliterated.  For  the  strike  of  the  cleavage 
coincides  more  or  less  closely  with  the  axes  of  folds  and  plications,  and 
is  thus  helpful  in  unravelling  the  geological  structure  of  a  complicated 
region. 

Regional  Metamorphism.— Not  much  need  be  said  on  the  subject  of 
mapping  an  area  in  which  regional  metamorphism  has  been  developed — 
the  structural  geology  being  frequently  highly  complicated  and  obscure, 
and  hardly  to  be  attempted  by  one  who  is  not  well  versed  in  petrography, 
and  has  had  little  experience  in  geological  surveying.  Nevertheless, 
even  a  beginner  will  find  much  to  interest  him  in  trying  to  puzzle  out  the 
structure  of  such  a  region. 

We  have  already  learned  that  regional  metamorphism  is  not  in- 
frequently a  result  of  deformation.  In  other  words,  the  rocks  of  such  a 
region  have  been  more  or  less  compressed  and  buckled  up  or  folded,  and 
in  many  places  have  yielded  to  tangential  squeezing  and  crushing, 
whereby  overthrusts  on  a  grand  scale  have  often  been  effected.  In 
mapping  an  area  which  exhibits  such  phenomena,  it  is  obviously  most 
important  that  we  should  be  able  to  lay  down  the  axial  lines  of  the  chief 
folds,  and  the  position  of  all  considerable  thrust-planes.  This  may  be 
done  without  troubling  ourselves  at  first  as  to  purely  theoretical  questions 
concerning  the  particular  chemical  and  mineralogical  changes  through 
which  the  rocks  may  have  passed.  It  is  more  than  likely,  however,  that 
as  we  proceed  with  our  field  observations  we  shall  be  confronted  with 
evidence  that  may  go  a  long  way  to  show  not  only  what  the  original 
character  of  the  rocks  may  have  been,  but  to  reveal  the  successive 
changes  which  some  of  them  have  undergone. 

Bearing  in  mind,  then,  that  the  rocks,  whatever  their  original  character 
may  have  been,  are  arranged  in  folds,  we  shall  expect  to  find  the  position 
of  these  indicated  by  the  outcrops  of  more  or  less  persistent  zones  or 
belts  of  different  kinds  of  schistose  rocks,  all  having  approximately  the 
same  trend.  These  bands,  we  may  safely  assume,  represent  the  general 
strike  of  the  series.  Not  infrequently,  however,  we  may  traverse  wide 
areas  throughout  which  only  a  monotonous  succession  of  one  and  the 
same  kind  of  rock  may  appear.  Nevertheless,  if  our  traverses  be 
sufficiently  extensive,  we  may  expect  ere  long  to  meet  with  other  types 
of  rock,  the  relative  position  of  which  will  enable  us  to  determine  the 
general  strike  or  alinement  of  the  whole  complex.  The  observer  must 
be  on  the  constant  outlook  for  bands  of  rock  which  are  characterised 
by  the  presence  of  minerals  peculiar  to  themselves.  Knowing  that  the 
production  of  these  minerals  is  due  in  all  probability  to  some  peculiarity 
in  the  composition  or  constitution  of  the  original  rocks,  their  presence 
may  sometimes  be  as  useful  in  working  out  a  stratigraphical  succession 
as  the  occurrence  of  fossils  in  a  series  of  unaltered  strata.  Beds  and 
bands  of  ores  not  infrequently  occur  in  connection  with  particular  kinds 
of  schist,  and  have  in  certain  regions,  as  in  Norway,  been  followed 


304  STRUCTURAL  AND  FIELD  GEOLOGY 

over  wide  areas,  and  as  these  ore-bearing  rocks  are  everywhere 
associated  with  the  same  kinds  of  schist,  there  can  be  no  doubt  that 
they  are  truly  stratiform,  and  indicate  a  definite  geological  horizon. 
Crystalline  limestones  and  dolomites  interbedded  with  certain  distinctive 
kinds  of  schist  have  in  like  manner  often  been  traced  for  long  distances, 
and  when  similar  calcareous  bands  accompanied  by  the  same  varieties 
of  schist  are  found  cropping  out  at  what  might  appear  to  be  either  lower 
or  higher  horizons,  the  probabilities  are  that  such  successive  outcrops 
are  simply  the  result  of  folding,  the  same  beds  coming  again  and  again 
to  the  surface. 

It  is  not  unlikely  that  the  observer,  while  traversing  a  region  of 
schistose  rocks,  may  occasionally  encounter  areas  of  less  highly  meta- 
morphosed strata.  He  may  be  able  to  recognise  well-marked  clastic 
rocks,  such  as  schistose  conglomerate,  quartz-rock,  phyllite,  greywacke, 
limestone,  etc.  Should  such  be  the  case,  the  strata  must  be  carefully 
followed  along  and  across  the  strike,  for  the  purpose  of  tracing  the 
changes  they  undergo  as  the  region  of  more  highly  metamorphosed  rock 
is  approached.  The  successive  bands  or  zones  of  distinctive  schists 
which  we  may  already  have  traced  through  this  latter  region,  we  may 
now  be  able  to  connect  with  particular  beds  occurring  in  the  area  of 
less  altered  rocks.  Should  such  be  the  case,  we  shall  have  no  reason 
to  doubt  that  the  schists  are  metamorphosed  sedimentary  strata  ;  and 
should  the  direction  of  their  foliation  coincide  with  the  dip  of  the 
individual  bands,  we  shall  be  justified  in  concluding  that  the  schistose 
structure  has  been  developed  along  original  planes  of  bedding.  This 
is  most  likely  to  be  the  case  when  isoclinal  folds  have  been  closely 
compressed,  so  that  the  rocks  are  either  on  end  or  disposed  in  highly 
inclined  positions.  When  the  folds  open  out,  foliation— often  following 
planes  of  cleavage — must  sometimes  have  coincided  with,  and  sometimes 
have  traversed,  the  original  bedding  at  various  angles.  Therefore,  the 
mere  direction  of  the  planes  of  foliation,  when  all  trace  of  bedding  has 
been  obliterated,  cannot,  in  the  absence  of  other  evidence,  be  relied  upon 
as  revealing  original  stratification. 

Just  as  the  observer  must  be  on  the  outlook  for  every  item  of  evidence 
that  seems  to  indicate  the  arrangement  of  metamorphosed  strata,  and 
to  reveal  the  original  character  of  the  beds,  so  he  must  endeavour  to 
ascertain  what  relation  the  eruptive  rocks  he  may  encounter  bear  to  the 
schists  they  traverse.  If  they  are  older  than  the  metamorphism,  then 
they  themselves  will  have  undergone  some  change,  and  may  be  as 
highly  crushed  and  foliated  as  the  schists.  If,  on  the  other  hand,  they 
are  of  later  date,  they  will  not  be  metamorphosed.  Possibly  the  geologist 
may  encounter  igneous  masses  of  older  date  than  the  metamorphism, 
which,  nevertheless,  have  a  normal  appearance.  When  such  masses, 
however,  are  followed  for  any  distance  they  will  doubtless  begin  to  show 
traces  of  crushing,  and  eventually  pass  into  schists  or  gneisses  as  they 
near  the  region  of  extreme  metamorphism. 

Both  normal  and  reversed  faults  may  appear  among  schistose  rocks — 
indeed,  faults  and  extensive  thrust-planes  may  almost  be  expected  to 


GEOLOGICAL  SURVEYING  305 

occur.  As  the  outcrops  of  thrust-planes  usually  follow  the  strike,  they 
are  sometimes  hard  to  detect  unless  they  be  on  a  grand  scale.  But  if 
the  observer  has  been  able  to  make  out  the  general  geological  structure, 
and  has  ascertained  that  the  schistose  rocks  show  a  more  or  less  definite 
succession,  careful  mapping  will  reveal  all  the  reversed  faults  of  any 
importance.  Frequently,  indeed,  these  give  rise  to  prominent  features 
at  the  surface,  following  as  they  do  some  determinate  direction  athwart 
the  face  of  mountain  slopes,  where  they  simulate  the  appearance 
of  horizontal  or  inclined  bedding-planes.  Thrust-planes  are  usually 
rendered  conspicuous  by  erosion.  Naturally,  they  often  bring  into 
juxtaposition  rocks  of  very  different  kinds — on  one  side,  it  may  be, 
massive  and  relatively  durable  rocks  ;  on  the  other  side,  more  readily 
disintegrated  and  degraded  materials.  Not  infrequently,  therefore, 
thrust-planes  are  laid  bare  by  the  removal  of  the  softer  rocks  from 
the  inclined  surface  of  harder  rocks  upon  which  they  rest.  Or,  in 
cases  where  a  gently  inclined  thrust-plane  has  brought  harder  or 
more  durable  rocks  to  rest  upon  less  resistant  rocks,  an  escarpment 
may  be  developed  by  erosion,  the  geological  structure  producing  the 
same  effect  as  the  intercalation  of  a  relatively  "hard"  bed  in  a  series 
of  "softer"  strata.  Occasionally,  running  water  has  hollowed  out  deep 
gullies  and  ravines  along  the  outcrops  of  thrust-planes  (Plate  XLII.). 
The  presence  of  a  considerable  thrust-plane  is  often  further  revealed 
by  the  crushed  and  brecciated  appearance  of  the  immediately  adjacent 
rocks.  So  shattered  may  the  rocks  be,  that  the  line  of  movement  often 
resembles  the  outcrop  of  a  breccia.  Still  more  notable  are  the  evidences 
of  metamorphism  induced  by  such  great  rock-displacements.  Clastic 
rocks,  for  example,  may  be  rendered  crystalline  and  schistose,  the 
foliation  extending  upwards  for  some  little  distance  above  the  plane 
of  rock-movement.  Massive  crystalline  eruptive  rocks  may,  in  like 
manner,  be  crushed  and  foliated,  while  ancient  gneissose  and  schistose 
rocks  become  similarly  modified,  new  planes  of  foliation  being  developed, 
which  may  cross  the  older  foliation  at  any  angle.  It  is  particularly 
noteworthy  that  the  younger  foliation  always  coincides  in  direction  with 
the  plane  of  rock-movement. 

The  system  of  thrust-planes  traversing  schistose  and  other  rocks  in 
a  region  of  highly  complicated  structure,  is  often  cut  across  by  one  or 
more  systems  of  normal  faults,  which  shift  the  thrust-planes  just  as  if 
they  were  outcrops.  Such  faults,  therefore,  can  be  detected  and  followed 
in  the  usual  way. 

Archaean  Rocks. — If  it  be  often  a  hard  matter  to  unravel  the  structure 
of  a  region  of  highly  metamorphosed  rocks,  it  is  still  more  difficult  to 
map  out  the  various  complicated  and  puzzling  phenomena  presented  by 
the  ancient  coarsely  banded  gneissose  rocks  that  seem  to  form  the 
foundation-stones  upon  which  the  oldest  sedimentary  strata  of  the 
globe  have  been  laid  down.  Hitherto,  all  attempts  to  work  out  the 
structure  and  succession  of  the  "Archaean  complex,"  as  developed  in 
particular  regions,  have  been  more  or  less  unsuccessful.  Now  and 
again,  what  may  at  first  appear  to  be  a  series  of  distinctive  kinds  of 

U 


306  STRUCTURAL  AND  FIELD  GEOLOGY 

gneiss,  alternating  the  one  with  the  other,  seems  to  suggest  a  possible 
chronological  succession.  But  this  apparent  order  rarely  continues  for 
any  distance.  Frequently,  one  of  the  gneissic  bands  will  break  across 
another — while  the  evidence  of  extreme  deformation  is  everywhere 
conspicuous.  The  belief  is  gaining  ground  that  these  ancient  rocks 
are  probably  for  the  most  part  of  deep-seated  igneous  origin — comparable 
to  those  batholiths  of  granite,  etc.,  with  their  associated  sheets,  dykes, 
and  veins,  which  have  given  rise  to  the  phenomena  of  contact  meta- 
morphism.  For  sheets  of  coarsely  banded  gneiss  not  only  cut  across 
similar  sheets  of  gneiss  and  beds  of  what  seem  to  be  metamorphosed 
sedimentary  rocks,  but  ever  and  anon  the  gneisses  lose  their  banded 
structure  and  graduate  into  amorphous  granitoid  masses.  Since  the 
time  or  times  of  their  extrusion,  however,  all  these  igneous  rocks  have 
been  subject  to  repeated  deformation,  and  dynamo-metamorphism  has 
modified  them  more  or  less  profoundly. 

Although  the  rocks  in  question  are  usually  grouped  under  the  general 
term  "Archaean,"  it  is  by  no  means  certain  that  they  all  belong  to  early 
pre-Cambrian  times.  It  is  quite  possible  that,  in  some  cases,  they 
may  represent  metamorphosed  sediments  of  early  Palaeozoic  age,  pierced 
in  all  directions  by  masses  and  sheets  of  intrusive  eruptives.  It  is 
obvious,  indeed,  that  unless  they  are  immediately  overlaid  by  rocks  of 
Cambrian  age,  their  pre-Cambrian  origin  cannot  be  demonstrated. 
Nevertheless,  the  general  similarity  of  the  rocks  of  the  so-called  "Archaean 
complex"  all  the  world  over,  is  suggestive.  But  the  mapping  of  these 
old  gneissoid  rocks,  and  the  interpretation  of  their  evidence,  are  beyond 
the  resources  of  the  beginner.  Not  without  much  patient  and  skilful 
work  in  the  field,  and  prolonged  investigation  in  the  laboratory,  will  the 
Archaean  rocks  give  up  their  secret. 


CHAPTER  XX 
GEOLOGICAL  SURVEYING — continued 

Mapping  of   Unconsolidated  Tertiary   Deposits,  and   of   Glacial    and 
Flu vio -glacial  Accumulations — Boulder-clay  ;   Roches  Moutonnees  ;* 
Terminal  Moraines,  etc.     Raised  Beaches.     Lacustrine  and  Fluvia- 
tile  Deposits.     Peat. 

Superficial  Accumulations. — In  this  and  other  countries 
the  "  solid  "  rocks  are  often  concealed  under  sheets  of  uncon- 
solidated  materials,  as  gravel,  sand,  loam,  clay,  etc.  Some- 
times these  superficial  accumulations  are  confined  to  valleys 
and  depressions,  or  they  may  mantle  the  entire  surface  of 
broad,  low-lying  lands.  They  are  of  very  various  origin — 
marine,  fluviatile,  lacustrine,  terrestrial — some  dating  back  to 
early  Tertiary  times,  while  others  belong  to  later  periods,  and 
many  are  still  in  process  of  formation. 

The  TERTIARY  deposits  of  this  country,  owing  to  their 
generally  unconsolidated  condition,  their  inconsiderable  thick- 
ness, and  limited  extent,  may  be  looked  upon  as  "  superficial 
accumulations."  They  are  chiefly  marine,  and  practically 
confined  to  circumscribed  areas  in  the  south-east  and  south 
of  England.  On  the  continent,  however,  they  cover  much 
more  extensive  areas — in  some  of  which  the  deposits  are 
essentially  of  marine  origin,  while  in  other  regions  they  are 
freshwater,  or  may  consist  of  an  alternation  of  marine  and 
freshwater  accumulations.  In  North  Germany,  Belgium, 
France,  and  England,  the  beds  are  arranged  in  approximately 
horizontal  positions — the  marine  and  fluvio-marine  deposits 
occurring  for  the  most  part  in  maritime  districts,  and  seldom 
reaching  more  than  a  few  hundred  feet  above  the  sea.  The 
deposits  vary  much  in  character — in  some  places  consisting 
largely  of  clay  or  marl,  in  other  places  of  sand  or  gravel. 

307 


308  STRUCTURAL  AND  FIELD  GEOLOGY 

These  old  sedimentary  formations,  since  the  time  of  their 
elevation,  have  been  subjected  to  very  considerable  erosion, 
but,  owing  to  their  generally  unconsolidated  character  they 
are  not  distinguished  by  any  very  prominent  surface  features 
— but  form,  for  the  most  part,  gently  undulating  low  grounds 
and  plains.*  The  mapping  of  such  accumulations  is  attended 
with  some  difficulty — it  being  often  hard  to  trace  the  outcrops. 
This  is  due,  in  the  first  place,  to  the  fact  that  upon  slopes  the 
junction-lines  are  obscured  by  the  washing  down  of  materials 
from  above — the  outcrops  of  lower  beds  being  often  entirely 
concealed  under  sand,  loam,  etc.,  derived  from  overlying  strata. 
Geologists  mapping  in  such  regions  occasionally  employ  a 
gouge-like  spud,  which  might  be  described  as  a  kind  of 
exaggerated  "cheese-taster,"  for  the  purpose  of  ascertaining 
the  position  of  the  concealed  outcrops  as  accurately  as 
possible.  The  annexed  diagram  will  serve  to  illustrate 
the  modus  operandi  (Fig.  1 20).  The  surface  from  x  to  b  shows 


FIG.  120.— CONCEALMENT  OF  OUTCROP  BY  SURFACE  WASH. 

Clay  (a)  overlaid  by  sand  (6). 

nothing  but  sand,  we  shall  suppose,  while  between  a  and  x 
clay  obviously  immediately  underlies  the  soil.  The  observer 
having  reason  to  believe  that  the  sand  at  x  and  for  some 

*  The  Tertiary  deposits  which  in  England  and  the  low  grounds  of 
Middle  Europe  generally  are  usually  unconsolidated  and  not  much 
disturbed — spreading  as  sheets  of  greater  or  less  thickness  over  Mesozoic 
and  older  rocks— are  represented  in  Southern  Europe  by  much  more 
massive  strata — the  older  portions  of  which  enter  largely  into  the  frame- 
work of  the  Pyrenees,  the  Alps,  the  Apennines,  etc.  It  would  be  an 
abuse  of  terms  to  speak  of  these  deposits  as  "superficial  formations." 
Even  in  this  country,  where  the  corresponding  deposits  are  of  slight 
thickness  and  more  or  less  unconsolidated,  they  are,  nevertheless,  not 
included  by  geologists  amongst  "superficial  formations  "—this  term 
being  restricted  to  post-Tertiary  and  recent  accumulations  alone.  From 
the  point  of  view,  however,  of  the  field  geologist,  all  loose  and  uncon- 
solidated beds  of  gravel,  sand,  clay,  loam,  etc.,  may  be  looked  upon  as 
superficial  accumulations. 


GEOLOGICAL  SURVEYING  309 

distance  up  the  slope  is  not  in  situ  but  remanie,  forces  his 
instrument  at  intervals  down  through  the  sand,  until  he 
reaches  a  place  where  his  borer  no  longer  touches  the  clay. 
Unless  the  amount  of  sand  carried  down  the  slope  be  very 
great,  it  is  obvious  that  the  observer  can  by  such  means 
attain  a  line  for  his  outcrop  which  cannot  be  far  from  the 
truth. 

In  low-lying  regions  of  Tertiary  deposits,  clear  natural 
sections  are  of  infrequent  occurrence — the  best  exposures 
being  met  with  along  sea-coasts,  and  in  recent  artificial 
cuttings  and  excavations.  Frequently,  indeed,  the  geologist 
is  largely  beholden  to  the  records  of  deep  well-borings,  etc.,  for' 
information  with  regard  to  the  succession  of  the  strata,  and 
the  probable  position  of  the  outcrops.  Many  hints  he  will 
doubtless  derive  from  a  careful  study  of  the  various  soils  and 
the  character  of  the  vegetation,  and  even  from  the  form  of 
the  ground.  Gravel,  for  example,  being  a  highly  porous 
deposit  rapidly  absorbs  rain,  and  is,  therefore,  less  liable  to 
be  washed  down  and  trenched  by  running  water,  while 
impervious  clay,  on  the  other  hand,  is  readily  attacked 
superficially.  The  former  deposit,  therefore,  will  often  tend 
to  form  dry  lands  with  a  gentle  or  more  rapidly  undulating 
surface.  Thick  sands,  in  like  manner,  will  give  rise  to  some- 
what similar  dry  rolling  ground ;  while  clays  may  form  low 
plains  or  higher  tracts  trenched  in  all  directions  by  running 
water.  But  in  countries  which,  like  our  own,  have  been  long 
under  cultivation,  the  soils  of  a  Tertiary  district  are  often  so 
transformed  that  it  is  hard  to  tell  from  them  what  the  precise 
nature  of  the  subjacent  deposits  may  be.  For  the  same 
reason,  plant-associations  in  such  areas  cannot  always  be 
trusted  as  guides  by  the  geological  surveyor.  Such  difficulties, 
however,  are  only  likely  to  happen  when  the  geologist  is 
dealing  with  the  outcrops  of  relatively  thin  accumulations — 
when,  on  the  other  hand,  a  stratum  or  series  is  thick  and 
covers  wide  areas,  the  nature  of  the  soil  and  the  character  of 
the  vegetation  will  help  the  observer  to  trace  its  extent  with 
more  or  less  confidence.  Speaking  in  general  terms,  we  may 
say  that  the  mapping  of  unconsolidated  Tertiary  deposits  is 
carried  on  in  much  the  same  way  as  the  tracing  of  consoli- 
dated sedimentary  strata.  Now  and  again  they  are  gently 


310  STRUCTURAL  AND  FIELD  GEOLOGY 

folded,  and  assume  a  basin-shaped  arrangement,  and  when 
such  is  the  case  the  outcrops  are  not  so  hard  to  trace. 

GLACIAL  AND  FLUVIO-GLACIAL  ACCUMULATIONS. — The 
deposits  included  under  this  head  are  widely  distributed  in 
this  country  and  in  corresponding  latitudes  of  the  Continent 
and  North  America.  They  cover  extensive  areas  in  our 
lowlands — occupying  valleys  and  sweeping  over  intermediate 
tracts,  so  as  largely  to  conceal  the  underlying  solid  rocks. 
In  our  mountainous  districts  they  are  mostly  restricted  to 
the  valleys,  but  often  extend  upwards  to  considerable  heights 
upon  the  mountains  themselves.  It  would  be  quite  beyond 
the  limits  of  this  work  to  attempt  any  detailed  description 
and  classification  of  these  accumulations.  Attention  is, 
therefore,  limited  to  some  of  the  salient  phenomena  presented 
by  the  more  notable  of  the  deposits  in  question. 

(a)  Boulder-clay  or  Till. — This  is  an  unstratified  or 
amorphous  mass,  the  essential  lithological  characters  of  which 
have  already  been  given  (see  p.  63).  One  of  its  most 
striking  peculiarities  are  the  stones  and  boulders  which  it 
contains.  These  are  almost  invariably  fresh,  unweathered, 
and  generally  blunted  and  subangular  in  shape — often  show- 
ing smoothed,  polished,  and  striated  faces.  The  beginner 
should  note  especially  the  character  of  the  striation  and  its 
relation  to  the  shape,  size,  and  species  of  the  stones.  Usually, 
stones  which  are  longer  than  broad  are  most  distinctly 
striated  lengthways,  while  those  which  are  as  broad  as  they 
are  long  are  striated  equally  in  all  directions ;  very  large 
blocks  are  often  smoothed  on  one  side  only,  while  smaller 
boulders  and  stones  are  commonly  smoothed  all  over ;  again, 
compact  fine-grained  rocks,  such  as  limestones,  shale,  iron- 
stone, felsite,  etc.,  have  usually  received  a  better  polish  than 
coarse-grained  grits,  sandstones,  etc.  The  observer  should 
be  on  the  outlook  for  any  traces  of  arrangement  of  the  stones 
and  boulders.  Occasionally,  lines  of  small  and  large  boulders 
may  be  seen  traversing  the  face  of  a  cutting  in  boulder-clay — 
the  boulders  not  infrequently  lying  lengthways.  Sometimes 
the  upper  surfaces  of  such  "  boulder-pavements,"  as  they  are 
termed,  are  distinctly  striated  in  one  common  direction.  The 
student  should  also  subject  the  gritty  clay  itself  to  a  close 
examination.  A  portion  should  be  taken  home  and 


GEOLOGICAL  SURVEYING  311 

thoroughly  dried  and  crumbled  down,  when  the  shape  and 
nature  of  the  larger  fragments  can  be  studied  with  the  help 
of  a  lens.  These  will  be  found  to  be  simply  minute  boulders, 
angular,  subangular,  and  often  striated,  and  quite  unweathered. 
The  "clay"  may  be  still  further  reduced  by  shaking  it  in 
water  and  passing  it  through  a  sieve.  By  using  sieves  of 
various  degrees  of  fineness,  all  the  gritty  particles  above  a 
certain  size  may  be  sifted  out,  and  only  an  extremely  fine- 
grained residue  be  left.  The  grit,  examined  microscopically, 
is  found  to  resemble  in  every  respect,  save  size,  the  small 
fragments  which  the  student  may  have  determined  with  the 
aid  of  his  pocket-lens.  They  all  alike  consist  of  fresh, 
unweathered  mineral  matter.  The  residue  which  is  not 
separated  by  the  finest  meshed  sieve  may  be  reduced  by 
shaking  it  in  water  and  allowing  it  to  settle.  From  the 
turbid  water  a  sediment  is  gradually  thrown  down.  The 
water  which  still  remains  clouded  can  then  be  decanted  and 
allowed  to  stand  until  it  clears.  In  this  way  we  obtain  a  still 
finer  grained  mechanical  precipitate.  These  sediments  are  of 
precisely  the  same  character  as  the  gritty  materials  separated 
by  the  sieve — they  are  fresh  and  unweathered,  being  com- 
posed of  what  may  be  termed  "rock-flour,"  the  chief 
constituent  of  which  is  powdered  quartz.  It  is  this  "  rock- 
flour  "  that  forms  the  major  portion  of  the  so-called  boulder- 
clay — the  proportion  of  true  clay  throughout  the  mass 
appearing  to  be  relatively  insignificant.  Indeed,  according 
to  Professor  Crosby,  "  till  in  its  natural  condition  is  often  less 
than  one-tenth  and  rarely  more  than  one-eighth  pure  clay." 

Boulder-clay  is  believed  to  be  the  bottom-  or  ground- 
moraine  of  massive  glaciers  or  ice-sheets — and  to  have  been 
formed  by  the  crushing  and  grinding  action  of  ice  in  motion. 
Formed  and  accumulated  under  these  conditions,  we  can 
readily  understand  why  it  should  consist  essentially  of  fresh, 
unweathered  rock-materials.  But  it  is  beyond  the  purpose 
of  these  notes  to  give  any  particular  account  of  this  remark- 
able formation.  It  may,  however,  be  of  service  to  the 
field-observer  to  indicate  certain  points  which  ought  to  be 
noted  when  he  begins  to  map  in  a  t ill-covered  region.  First, 
then,  the  configuration  of  the  surface  should  be  considered. 
Sometimes  the  ground  in  such  a  region  is  devoid  of  any 


312  STRUCTURAL  AND  FIELD  GEOLOGY 

prominent  feature,  rising  and  falling  in  long,  gentle  undula- 
tions, that  may  not  trend  in  any  particular  direction.  In 
other  cases,  the  surface  is  more  diversified,  and  may  show  a 
pronounced  "corduroy"  or  wrinkled  configuration — being 
marked  by  a  series  of  longer  and  shorter  parallel  and  often 
interosculating  banks  with  intervening  hollows.  The  trend  of 
these  drums,  or  drumlins  as  they  are  called,  should  be 
carefully  noted.  In  most  cases  the  banks  in  question  appear 
to  be  original,  i.e.  they  are  forms  assumed  by  the  boulder- 
clay  while  it  was  being  accumulated.  Occasionally,  however, 
they  are  simply  the  result  of  the  unequal  erosion  of  a  gently 
undulating  or  plain-like  surface  of  boulder-clay. 

The  colour  of  the  till  and  the  nature  of  its  included  stones 
and  boulders  should  be  noted.  The  colour  will  usually  be 
found  to  correspond  with  that  of  the  predominant  rock  or 
rocks  of  a  district — it  is  therefore  local.  The  most  abundant 
rock-fragments  in  the  till  are  also  generally  local,  but  com- 
mingled with  these  many  others  of  more  distant  derivation 
are  sure  to  occur.  The  observer  should  take  percentages  of 
the  different  kinds  of  rock,  and  endeavour  to  ascertain  their 
several  sources.  If  he  be  a  beginner  he  will  naturally  be  at 
fault,  but  a  good  geological  map  of  the  country  will  afford  him 
some  help,  and  he  may  obtain  more  by  examining  the  rock- 
collections  in  public  museums.  Should  he  be  able  to  deter- 
mine the  source  of  many  of  the  stones  which  are  "  strangers," 
this  will  give  him  a  strong  hint  as  to  the  general  direction 
followed  by  the  old  mer  de  glace. 

Lenticular  layers  and  sometimes  thicker  series  of  un- 
fossiliferous  gravel,  sand,  and  laminated  clay,  may  occur 
underneath,  and  are  still  more  frequently  included  in  boulder- 
clay.  Such  deposits  are  often  more  or  less  confused  and 
disturbed.  They  obviously  point  to  the  action  of  subglacial 
water.  The  boulder-clay  that  immediately  underlies  them 
will  be  found  quite  fresh  and  unaltered,  showing  that  it  has 
never  been  exposed  to  the  oxidising  influence  of  the  atmos- 
phere. Now  and  again,  however,  stratified  deposits  of  gravel, 
sand,  loam,  marl,  peat,  etc.,  are  met  with  resting  upon  and 
covered  by  boulder-clay.  The  boulder-clay  underneath  such 
beds  is  almost  invariably  discoloured  for  some  distance  down- 
wards, thus  showing  that  it  must  for  some  time  have  been 


[7'o  /ace  page  312. 


GEOLOGICAL  SURVEYING  313 

acted  upon  by  the  atmosphere  and  surface  water.  The 
stratified  deposits  in  question  have  often  yielded  relics  of  an 
old  land-surface,  and  are  thus  evidence  that  the  formation  of 
boulder-clay  was  an  interrupted  and  not  a  continuous  process. 
The  same  inference  may  be  drawn  from  the  occurrence  of 
marine  fossiliferous  deposits  included  in  till. 

The  relation  of  the  boulder-clay  to  the  immediately  sub- 
jacent rocks  is  deserving  of  study.  The  latter  are  sometimes 
so  broken,  jumbled,  and  confused,  that  it  is  hard  to  say  where 
the  shattered  and  disturbed  rock  ends  and  boulder-clay 
begins.  The  student  should  note  whether  the  slabs  and  reefs 
of  rock  have  been  bent  over  or  wedged  out  of  their  beds,  and 
the  direction  in  which  they  have  moved  should  be  ascertained. 
Instead  of  being  broken  and  jumbled,  the  subjacent  rocks 
may  show  a  smoothed,  polished,  and  striated  surface.  The 
compass  bearing  of  the  striae  should  always  be  taken,  as  this 
indicates  precisely  the  direction  of  ice-flow  at  the  point  of 
observation.  It  is  possible  that  the  beginner  may  at  first 
have  some  difficulty  in  distinguishing  between  a  glacially 
striated  surface  (Plate  LII.)  and  slickensides  (Plate  XXXVI.). 
The  latter,  however,  are  usually  confined  to  flat  or  even 
surfaces,  and  are  frequently  glazed  with  mineral  matter — the 
scratches,  moreover,  are  strictly  parallel.  It  will  be  noted 
further  that  when  a  slickensided  surface  shows  any  depres- 
sions these  are  not  striated.  Glacial  striae,  on  the  other  hand, 
may  occur  on  flat,  convex,  concave,  or  rapidly  undulating 
surfaces.  The  smoothing  and  polishing  is  not  confined  to 
the  protuberances  upon  a  rock-surface,  but  every  little  dimple 
and  depression  is  equally  dressed.  Although  roughly  parallel, 
glacial  striae  are  yet  not  so  straight  as  slickensides,  but  often 
cross  each  other  at  acute  angles.  Frequently,  indeed,  they 
may  be  seen  curving  gently  round  the  sides  of  projecting 
knobs,  as  if  these  had  caused  some  slight  deviation  of  the  ice- 
flow.  The  scratches  may  be  as  fine  as  if  drawn  by  an 
engraver's  needle,  or  they  may  be  coarse,  jagged  ruts ;  and 
between  these  extremes  all  gradations  occur,  and  may  be  seen 
side  by  side  on  the  same  rock-face. 

Roches  moutonntes. — Dressed  rock-surfaces  occur  not  only 
underneath  boulder-clay,  but  on  exposed  hill-slopes  and  rocky 
elevations,  from  which  the  boulder-clay  has  been  stripped  by 


314  STRUCTURAL  AND  FIELD  GEOLOGY 

denudation,  and  in  many  places  also  where  probably  no 
boulder-clay  was  ever  deposited.  The  observer  should  take 
particular  note  of  the  configuration  of  the  hills  and  mountains 
of  a  glaciated  region  (see  Plate  LI  1 1.).  Land  which  has 
been  subjected  to  extreme  glaciation  generally  shows  flowing 
contours.  Projecting  prominences  and  crags  are  smoothed 
and  rounded  off  on  the  side  that  faced  the  ice-flow ;  while  the 
opposite  side,  protected  by  its  position,  may  retain  its 
original  roughness.  The  smoothed  face  is  termed  the  Stoss- 
seite,  and  the  non-glaciated  face,  the  Lee-seite.  Whole 
mountain-slopes  may  exhibit  a  rudely  mammilated  surface, 
the  rounded  rock-surfaces  being  often  striated.  Sometimes 
the  ice-markings  are  fresh  and  readily  recognisable ;  at  other 
times  they  have  almost  vanished — the  mere  "ghosts  of 
scratches."  Even  when  they  have  disappeared,  however,  the 
mammilated  outlines  of  the  rock-masses  are  cogent  evidence 
of  the  former  presence  of  glacier-ice.  These  rounded  hum- 
mocks or  roches  rnoutonne'es,  as  they  are  called,  generally 
indicate  clearly  enough  the  direction  of  ice-flow.  In  the  case 
of  abrupt  crags  and  tors  the  stoss-seite  is  usually  steep,  while 
the  lee-seite  assumes  the  form  of  a  long  sloping  ridge.  This 
phenomenon  is  known  as  crag-and-tail.  Sometimes  the  tail 
is  composed  entirely  of  glacial  detritus  (boulder-clay,  gravel, 
etc.).  More  frequently  (especially  when  the  crag  is  very 
prominent  and  of  considerable  dimensions)  the  "tail" 
consists  largely  of  solid  rock,  usually  covered  with  a  less  or 
greater  thickness  of  boulder-clay,  etc. 

Terminal  Moraines:  Perched  Blocks,  etc. — In  many  of 
our  mountain  valleys,  angular  blocks  and  earthy  debris  are 
sprinkled  more  or  less  abundantly  over  the  ground,  up  to 
very  considerable  elevations.  In  the  bottoms  of  the  valleys, 
knobby  ridges,  mounds,  and  hillocks,  composed  of  the  same 
materials,  are  of  common  occurrence.  The  character  of  the 
deposits,  the  peculiar  shape  of  the  hillocks,  etc.,  and  their 
position,  are  comparable  in  all  respects  to  similar  phenomena 
occurring  in  the  glacier-valleys  of  alpine  regions.  There  can 
be  no  doubt  that  they  are  the  terminal  moraines  of  extinct 
glaciers.  Low  ridges  or  banks,  or  lines  of  morainic  debris 
running  along  the  mountain-slopes  of  many  highland  valleys, 
correspond  to  the  lateral  moraines  of  existing  glaciers.  Perched 


I 


[To  /ace  pag^e  314. 


GEOLOGICAL  SURVEYING  315 

blocks  are  erratics  which  have  been  carried  by  the  ancient 
glaciers,  and  successively  stranded  as  the  great  ice-flows 
melted  away. 

Sheets  and  Mounds  of  Gravel  and  Sand. — Throughout 
wide  areas,  boulder-clay  is  often  more  or  less  deeply  buried 
under  gravel  and  sand.  These  deposits  may  assume  the 
form  of  extensive  sheets  with  a  gently  undulating  surface ; 
or  they  may  occur  as  long  curving  and  irregularly  winding 
ridges  ;  or  as  tumultuous  groups  of  closely  associated  mounds, 
hummocks,  and  ridges,  known  as  kames.  Sometimes  the  ac- 
cumulations consist  principally  of  fine  sand — often  diagonally 
bedded — or  of  interbedded  sand  and  laminated  clay.  In 
other  places,  sand  and  gravel  are  equally  prominent,  while 
elsewhere  coarse  shingle  and  boulders  are  most  abundant. 
At  rarer  intervals  a  mound  or  ridge  may  be  composed  of  rude 
morainic  debris — a  rubble  of  angular  blocks  and  rock-rubbish. 


FIG  121. — COARSE  GRAVEL  AND  SHINGLE,  SHOWING  IMBRICATED 
STRUCTURE. 

The  arrow  indicates  the  direction  of  the  current. 

It  is  obvious  that  deposits  so  heterogeneous  could  not 
all  have  been  formed  in  quite  the  same  way ;  and  it  would 
be  out  of  place  here  to  discuss  the  various  views  which  have 
been  entertained  as  to  their  origin.  The  general  belief, 
however,  is  that  all  the  deposits  in  question  were  accumulated 
while  the  old  icy  covering  of  the  land  was  gradually  melting 
away.  The  observer  will  note  that  the  long  winding  ridges 
(known  as  eskers)  are  composed  chiefly  of  gravel — often 
very  coarse — with  more  or  less  numerous  boulders.  They 
have  obviously  been  laid  down  by  torrential  water — and 
when  good  sections  across  an  esker  are  exposed,  the  stones 
sometimes  show  that  imbricated  arrangement  which  one  may 
often  observe  amongst  the  stones  and  coarse  shingle  of 
streams  and  rivers  (Fig.  121).  Many  geologists  incline  to  the 
belief  that  these  eskers  mark  the  sites  of  subglacial  torrents 


316  STRUCTURAL  AND  FIELD  GEOLOGY 

by  which  the  great  mers  de  glace  were  tunnelled,  especially 
during  the  period  of  their  final  dissolution.  In  the  low- 
lying  parts  of  the  country,  many  wide  sheets  of  sand  and 
gravel  seem  also  to  have  been  accumulated  underneath  the 
melting  ice-flows,  for  they  are  often  closely  associated  with 
eskers.  In  other  cases,  however,  they  may  have  been  dis- 
tributed over  the  exposed  surface  of  the  low  lands  by  water 
escaping  from  the  gradually  disappearing  snow-fields  and 
decaying  glaciers  of  adjacent  high  grounds. 

In  hilly  and  mountainous  tracts,  narrow  and  broad 
terraces  and  considerable  plateaus  of  gravel,  sand,  and  clay 
obviously  mark  the  sites  of  ancient  glacier-lakes.  Such  are 
the  Parallel  Roads  of  Glenroy,  the  wide  sand-plains  covering 
the  watershed  between  the  rivers  Avon  and  Irvine  in  the 
neighbourhood  of  Loudoun  Hill,  and  many  similar  terraces 
and  flats  occurring  in  the  Northern  Highlands  and  Southern 
Uplands.  Even  the  hill-slopes  overlooking  the  broad  low- 
land tracts  of  Scotland  now  and  again  show  strong  evidence 
of  the  former  presence  of  temporary  glacial  lakes,  which 
appear  to  have  come  into  existence  after  the  hills  in  ques- 
tion had  been  divested  of  their  icy  covering,  and  while  the 
adjacent  lowlands  were  still  thickly  mantled  by  the  gradually 
decaying  mer  de  glace. 

There  seems  also  'to  be  little  doubt  that  those  tumultuous 
assemblages  of  hummocks,  cones,  and  ridges  known  as  kames, 
are  of  the  nature  of  gravelly  moraines,  deposited  in  front  of 
giant  glaciers  or  district  ice-sheets.  Often  associated  with 
them  are  wide  sheets  of  sand,  loam,  and  clay,  which  spread 
out  over  the  low-lying  tracts,  upon  the  borders  of  which  the 
gravelly  moraines  have  been  accumulated.  Perhaps  one  of 
the  best  areas  for  the  study  of  these  phenomena  is  the  great 
valley  of  Strathmore. 

Although  the  external  form  of  glacial  and  fluvio-glacial 
deposits  is  often  original,  yet  occasionally  widespread  sheets 
of  sand,  gravel,  etc.,  have  been  so  cut  up  by  subsequent 
epigene  action  as  to  present  a  more  or  less  rapidly  undulating 
or  corrugated  surface.  When  this  is  the  case,  such  a  denuded 
plain  now  and  then  simulates  the  appearance  of  "  drums " 
and  "kames."  Usually,  however,  the  observer  is  not  likely 
to  mistake  a  surface  of  erosion  for  one  of  accumulation.  If 


GEOLOGICAL  SURVEYING 

the  deposits  consist  of  evenly  bedded  gravel,  sand,  etc.,  the 
subsequent  origin  of  the  mounds  and  banks  will  be  disclosed 
by  the  manner  in  which  the  horizontal  beds  are  truncated 
by  the  slopes  of  the  ground.  It  is  more  difficult  to  differ- 
entiate between  a  true  "  drum  "  or  "  drumlin " — that  is,  a 
bank  or  ridge  of  boulder-clay  due  to  glacial  accumulation, 
and  banks  of  the  same  material  which  have  resulted  from  the 
irregular  erosion  of  a  thick  continuous  sheet.  If  the  banks 
do  not  trend  in  the  same  direction  as  the  roches  moutonnees 
and  striae  of  a  district,  then  they  are  not  true  drumlins. 
Should  their  trend  appear  to  coincide  generally  with  that 
of  glaciation,  the  whole  modelling  of  the  surface  must  be 
studied  before  we  come  to  the  conclusion  that  the  banks 
are  original  structures.  If  they  have  been  carved  out  of  a 
sheet  of  boulder-clay  by  running  water,  evidence  of  this 
should  be  found  in  the  arrangement  of  the  intervening 
hollows,  which  will  be  grouped  much  in  the  same  way  as 
the  feeders  and  tributaries  of  a  stream.  In  other  words,  the 
banks  and  ridges  of  the  district  will  not  be  arranged  through- 
out in  parallel  positions,  but  will  fan-out  as  they  are  followed 
in  a  direction  opposite  to  that  of  the  water-flow.  Moreover, 
the  existing  brooks  and  their  feeders,  or,  should  these  have 
disappeared,  the  evidence  of  their  former  presence  afforded 
by  flats  and  terraces  of  alluvial  deposits  occupying  the 
hollows,  would  be  sufficient  to  convince  us  that  the  banks 
or  ridges  were  not  true  drumlins,  but  secondary  structures. 
It  is  to  be  noted,  however,  that  true  drumlins  are  of  two 
kinds — while  some  have  been  accumulated  as  such  under- 
neath the  old  ice-sheets,  others  would  appear  to  be  merely 
the  remnants  of  widespread  sheets  of  boulder-clay  which 
have  been  exposed  to  subsequent  glacial  erosion.  In  Galloway, 
for  example,  the  low  grounds  extending  outwards  from  the 
mountains  were  originally  deeply  covered  with  extensive 
sheets  of  boulder-clay,  by  the  mer  de  glace  that  formerly 
overflowed  all  that  region.  Long  after  the  disappearance  of 
that  ice-sheet,  great  glaciers  streamed  out  from  the  mountain- 
valleys  for  some  little  distance,  and  trenched  and  furrowed 
the  older  boulder-clay — thus  forming  a  series  of  secondary 
drumlins. 

RAISED  BEACHES. — These  are  flats  and  terraces  occurring 


318  STRUCTURAL  AND  FIELD  GEOLOGY 

at  various  levels  above  the  sea  (see  Plate  LIV.  i).  They 
may  consist  of  ordinary  beach  materials — gravel  and  sand 
with  rolled  and  broken  sea-shells,  etc.  Along  the  margins  of 
estuaries  they  often  form  wide  flats,  composed  for  the  most 
part  of  finer  materials — sand,  clay,  loam,  silt,  etc.  On  our 
more  exposed  sea-coasts  the  raised  beach-lines  are  frequently 
mere  platforms  and  ledges,  which  have  been  sawn  out  of  the 
solid  rocks  by  the  sea.  Many  old  beaches  are  backed  by  cliffs, 
at  the  base  of  which  sea-worn  caves  not  infrequently  appear. 
In  Scotland  there  are  three  "ancient  sea-margins"  which  are 
particularly  noteworthy.  They  occur  at  heights  of  100  feet, 
50  feet,  and  -25  feet  respectively.  The  highest  is  the  oldest, 
and  is  best  developed  in  the  basins  of  the  Forth,  the  Clyde, 
and  the  Tay.  It  is  composed  largely  of  laminated  brick-clay, 
together  with  fine  sand.  Scattered  through  these  deposits 
occasional  stones  occur,  and  now  and  again  large  erratics  are 
even  common.  The  beds  not  infrequently  yield  Arctic 
species  of  shells,  etc.  The  observer  will  find  it  interesting  to 
follow  the  100  feet  beach  or  terrace  up  the  valleys  until  it 
merges  into  terraces  of  ordinary  fluviatile  shingle  and  gravel. 
When  the  latter  are  traced  further  inland  into  the  mountains, 
they  will  eventually  be  found  to  interosculate  with  fluvio- 
glacial  gravels  and  terminal  moraines. 

The  two  lower  terraces  are  of  later  date,  but  their  geo- 
logical history  has  not  yet  been  so  fully  worked  out  as  it 
deserves  to  be. 

LACUSTRINE  AND  FLUVIATILE  DEPOSITS.— The  sites  of 
old  lakes  are  readily  detected.  They  invariably  occur,  as  might 
have  been  expected,  in  hollows  and  depressions,  and  usually 
form  level  meadow-lands.  Their  margins,  as  a  rule,  are  well 
defined.  The  observer  should  never  miss  the  opportunity  of 
examining  any  cuttings  in  which  the  old  lacustrine  deposits 
are  exposed.  Very  often  the  immediate  surface  is  occupied 
by  peat  of  less  or  greater  thickness — or  several  layers  of  peat 
may  be  interstratified  with  sand  or  silt.  The  peat  may 
consist  entirely  of  plants  which  still  grow  in  the  neighbourhood. 
Now  and  again,  however,  Arctic  plants  have  been  detected  in 
the  basal  part  of  the  peat  or  in  the  immediately  underlying 
silt  or  clay.  Sometimes,  also,  traces  of  Arctic  animal  life  are 
met  with  in  the  same  deposits.  This  proves  that  some  of 


LIV. 


i.  RAISED  BEACHES,  NEAR  ELIE,  FIFE. 

Photo  by  Dr  Laurie. 


2.  Moss  NEAR  YELLOW  TOMACH,  MERRICK 

Photo  by  Mr  F.  J.  Lewis. 


[To  face  page  318. 


GEOLOGICAL  SURVEYING  319 

these  ancient  lakes  date  back  to  the  glacial  period.  Even 
those  lacustrine  deposits  which  are  entirely  of  post-glacial  age, 
have  often  yielded  interesting  fossils — amongst  which  may  be 
mentioned  remains  of  the  ancient  types  of  oxen  (Bos  primi- 
genius,  B.  longifrons),  the  great  Irish  deer,  wolf,  beaver,  etc., 
not  to  mention  relics  of  prehistoric  man.  Freshwater  shells 
also  frequently  occur,  forming  beds  of  shell-marl. 

Very  few  broad  river-valleys  fail  to  show  old  terraces  of 
gravel,  sand,  etc.,  occurring  at  various  levels  above  the  present 
streams.  These  mark  levels  at  which  the  rivers  formerly 
flowed.  Terraces  of  this  kind  are  best  developed  in  valleys 
which  have  been  more  or  less  abundantly  clothed  with  glacial 
and  fluvio-glacial  deposits  :  or  in  regions  where  the  rocks  have 
yielded  not  less  readily  to  fluviatile  erosion.  In  a  country 
like  Scotland,  where  the  rocks  are  all  relatively  "  hard,"  old 
river-terraces  may  be  said  never  to  occur  outside  of  preglacial 
valleys.  So  long  as  our  rivers  follow  their  preglacial 
courses,  those  terraces  are  almost  invariably  in  evidence — the 
rivers  making  their  way  through  broad  open  valleys.  No 
sooner,  however,  does  a  stream  leave  its  preglacial  course  to 
cut  its  way  through  the  older  rocks,  than  the  whole  character 
of  the  valley  changes.  The  stream  no  longer  flows  through 
a  wide  terrace-fringed  valley,  but  through  a  relatively  narrow 
ravine. 

PEAT. — Reference  has  been  made  to  the  occurrence  of 
peat  in  old  lacustrine  depressions.  But,  as  everyone  knows, 
peat  often  covers  wide  areas  of  rolling  low  ground  and  high 
plateau,  and  even  swathes  relatively  steep  mountain  slopes. 
In  some  regions,  indeed,  it  is  found  capping  flat  hill-tops. 
There  is  no  difficulty  in  mapping  peat-bogs,  but  a  careful 
study  of  their  phenomena  has  still  to  be  made.  It  is  well 
known  that  many  peat-bogs  cover  and  conceal  the  stumps 
and  stools  of  trees  which  are  rooted  in  an  ancient  soil,  and 
obviously,  therefore,  grew  in  situ  (see  Plate  LIV.  2).  Not 
only  so,  but  deep  cuttings  in  certain  peat-bogs  have  revealed 
the  presence  of  one  or  more  such  old  "  forest-beds  "  occurring 
one  above  another  in  the  peat  itself.  Scandinavian,  Danish, 
and  German  observers  have  detected  in  the  peat-bogs  of 
Northern  Europe  similar  phenomena,  and  have  gathered 
much  additional  botanical  evidence  of  varying  climatic 


320  STRUCTURAL  AND  FIELD  GEOLOGY 

conditions.  Until  recently  few  attempts  had  been  made  by 
competent  botanists  to  subject  the  peat-bogs  of  this  country 
to  a  like  careful  examination.  Geologists  specially  interested 
in  the  later -chapters  of  the  stony  record  have  for  a  long  time 
believed  that  a  rich  harvest  of  results  would  yet  be  reaped  in 
this  promising  field  of  inquiry.  The  purely  geological 
evidence  seemed  to  lead  to  the  conclusion  that  the  peat-bogs 
with  their  associated  "  forest-beds "  belonged  to  a  period 
during  which  several  well-marked  alternations  of  climate  took 
place — the  peat  being  the  product  of  wet  and  cold  conditions, 
while  the  "  forest-beds  "  indicated  relatively  dry  and  temperate 
conditions.  The  results  recently  obtained  by  Mr  F.  J.  Lewis, 
in  his  botanical  investigations  into  the  composition  and 
structure  of  our  peat-bogs,  have  abundantly  confirmed  that 
conclusion.  He  has  discovered  distinct  zones  of  Arctic  plants 
in  the  peat  of  lowlands  and  highlands  alike,  and  thus  we  can 
no  longer  doubt  that  the  closing  stages  of  the  geological 
history  of  our  islands  were  characterised  by  alternations  of 
cold  and  temperate  climatic  conditions.* 

*  For  a  general  account  of  Mr  Lewis's  investigations  see  Science 
Progress,  Vol.  II.,  p.  307. 


CHAPTER   XXI 

GEOLOGICAL   MAPS   AND   SECTIONS 

Geological  Maps  and  Explanatory  Memoirs.  Geological  Sections- 
Horizontal  or  Profile  Sections  should  show  both  the  Form  of  the 
Ground  and  the  Geological  Structure  ;  Direction  in  which  such 
Sections  should  be  drawn  ;  Method  of  plotting  a  Section  on  a  True 
Scale.  Vertical  Sections. 

Geological  Maps  and  Explanatory  Memoirs. — The  accom- 
panying maps  (Plates  LV.,  LVI.)  will  serve  to  illustrate  the 
method  of  tracing  geological  lines,  some  account  of  which 
has  been  given  in  the  two  preceding  chapters.  In  Plate  LV. 
only  the  actual  rock-exposures  seen  by  the  geologist  are 
represented.  These  are  indicated  by  the  arrows  and  con- 
tinuous lines,  the  patches  of  colour  showing  the  extent  of  the 
areas  where  rocks  come  to  the  surface.  Plate  LVI.  repre- 
sents the  same  region  with  the  several  boundary-lines,  etc., 
completed.  The  direct  and  indirect  evidence  which  guides 
the  observer  in  carrying  outcrops,  faults,  etc.,  from  one  point 
to  another,  must  no  doubt  be  of  unequal  value.  In  some 
places  it  may  be  so  convincing  that  one  may  almost  feel 
justified  in  representing  outcrops,  etc.,  by  means  of  continuous 
lines,  while  in  other  places  it  may  be  so  slight  that  the 
boundaries  laid  down  upon  the  map  can  be  only  an 
approximation  to  the  truth,  and  should,  therefore,  be  indicated 
by  dotted  or  interrupted  lines.  A  glance  at  the  maps  will 
show  that  the  geological  structure  of  the  region  represented 
is  so  devoid  of  complexity  and  the  evidence  so  full,  that  the 
observer  could  hardly  go  far  astray  in  carrying  lines  across 
such  a  country.  The  maps  are  merely  diagrams,  however, 
designed  to  bring  into  one  view  certain  leading  structures  and 
the  method  of  tracing  these,  and  therefore  it  must  not  be 
321  X 


322  STRUCTURAL  AND  FIELD  GEOLOGY 

inferred  that  outcrops  can  be  always  so  satisfactorily  followed. 
Sometimes,  indeed,  the  ground-rocks  are  so  obscured  over 
wide  areas,  by  superjacent  glacial  or  alluvial  accumulations, 
that  the  general  geological  structure  can  only  be  surmised, 
and  the  following  of  outcrops  becomes  impossible.  At  other 
times  the  region  may  be  so  abundantly  faulted  that  no 
cautious  geologist  would  venture  to  continue  an  outcrop 
beyond  the  point  where  the  rock  itself  could  be  proved  to 
exist.  Occasionally,  however,  in  the  case  of  valuable  rocks 
and  minerals  (coal,  ironstone,  etc.),  the  observer,  desirous  of 
helping  the  mining  engineer  in  his  search  for  such  seams, 
might  be  justified  in  indicating  by  means  of  interrupted  lines 
the  general  course  which  he  thought  the  outcrops  were  likely 
to  take.  But  in  doing  so,  he  would  be  careful  to  note  upon 
the  map  or  in  his  description  that  the  outcrops  were  largely 
conjectural,  and  therefore  not  to  be  implicitly  trusted. 

Three  geological  systems  are  represented  on  the  diagram- 
map — Silurian  (S),  Carboniferous  (C),  and  Jurassic  (J).  Each 
of  these  systems,  we  shall  suppose,  has  yielded  to  the 
investigator  its  assemblage  of  type-fossils.  It  will  be 
observed,  however,  that,  even  in  the  absence  of  fossils,  the 
geologist  could  have  had  no  difficulty  in  detecting  the 
presence  of  three  distinct  series  of  strata,  and  in  ascertaining 
the  order  of  their  succession.  The  rock-exposures  are  so 
numerous  that  they  at  once  reveal  the  geological  structure. 
The  series  marked  J,  for  example,  rests  with  a  strong  uncon- 
formity upon  the  series  C ;  the  latter  bearing  in  like  manner 
a  similar  relation  to  the  series  S.  Another  unconformity  of 
less  importance  occurs  in  the  Carboniferous  series,  where 
the  upper  group  (C2)  is  represented  as  gradually  stealing 
across  the  outcrops  of  the  lower  group  (C1) — the  structure, 
indeed,  is  a  combination  of  overlap  and  unconformity.  The 
accompanying  section  is  taken  along  the  line  A — B,  and  gives 
a  view  of  the  general  geological  structure. 

In  his  monograph  or  explanatory  memoir  of  such  a 
region,  the  geologist  would  probably  begin  by  sketching  its 
physical  features,  after  which  he  would  proceed  to  give  some 
account  of  the  general  distribution  of  the  .several  systems, 
and  their  relation  to  one  another.  Next  would  follow  a 
particular  description  of  each,  beginning  with  the  oldest. 


PLATE    LV. 


Place  between  pages   322  &  323] 


PLATE    LVI. 


GEOLOGICAL  MAPS  AND  SECTIONS  323 

The  several  faults  and  the  eruptive  rocks  would  likewise  call 
for  ample  notice.  In  short,  every  detail  of  scientific  interest 
and  economic  importance  would  be  duly  set  forth  in  its 
proper  place.  But  if  his  monograph  were  written  for  experts 
only,  the  geologist  would  necessarily  leave  much  unsaid, 
knowing  that  his  readers  might  be  relied  upon  to  fill  in  out- 
lines and  to  draw  obvious  inferences  for  themselves.  He  might, 
indeed,  not  infrequently  content  himself  with  a  bare  narration 
of  facts,  and  leave  these  and  his  map  to  tell  their  own  tale. 
Interpretations  and  explanations  would  only  be  called  for 
in  cases  where  the  evidence  was  incomplete  or  not  quite 
clear. 

The   beginner,   however,   who   essays   to    work    out   the 
geological  structure  of  a  district,  would  do  well  to  ponder 
over  the  evidence  as  it  grows,  and  endeavour  to  realise  the 
particular   conditions    under   which   the   various    rocks    and 
rock-structures  originated.     His  object  is  not  only  to  make 
a  correct  geological  map  and  to  present  a  detailed  report  of 
what  he  has  observed,  but  to  picture  to  himself  as  clearly  as 
he  can  the  succession  of  changes  which  have  taken  place  in 
the  region  surveyed.     If  he  is  dealing  with  sedimentary  strata, 
he  must  be  on  the    alert   to  notice  every  variation  in  the 
character  of  the  deposits,  every  fact  that  would  seem  to  throw 
light  upon  the  conditions  that  obtained  at  the  time  the  strata 
were   being   accumulated.      As   the   evidence   furnished    by 
fossils  is  always  most  important,  he  will  be  on  the  constant 
outlook  for  these.     Only  by  keeping  each  kind  of  evidence — 
that  of  the  fossils  and  that  of  the  rocks  themselves — con- 
stantly in  view,  can  we  hope  to  read  geological  history  aright. 
If  we  have  previously  made  ourselves  well  acquainted  with 
the  nature  and  mode  of  formation  of  sediments  now  being 
laid  down  in  lakes,  estuaries,  and  seas,  and  have  acquired  a 
sufficient  knowledge  of  the  various  ways  in  which  organic 
remains  come  to  be  entombed,  we  shall  be  prepared  to  give 
.  a  good  account  of  any  series  of  sedimentary  strata  we  may 
encounter.     Most  of  the   fossils  we   may  detect   will  in  all 
likelihood   belong  to   well-known  genera — probably  most  of 
the  species  themselves  will  already  have  been  recognised  by 
palaeontologists — so  that  with  the  combined  evidence  of  rocks 
and  fossils  the  observer  will  be  in  a  position  to  realise  the 


324  STRUCTURAL  AND  FIELD  GEOLOGY 

conditions  under  which  the  fossil-bearing  beds  were  laid 
down.  He  should  be  able,  in  short,  to  summon  up  a  picture 
of  the  past.  The  more  fully  he  has  stored  his  mind  with  a 
knowledge  of  geological  changes  now  in  operation,  and  the 
more  consistently  he  applies  this  knowledge  towards  the 
interpretation  of  the  stony  record,  the  better  investigator 
must  he  become,  and  the  more  clearly  and  vividly  will  the 
dead  past  live  again  for  him.  It  is  this  clothing  of  the  dry 
bones  with  flesh,  this  reconstruction  of  long-vanished  lands 
and  seas,  this  repeopling  of  the  world  with  types  of  life  that 
have  passed  away  for  ever,  this  gradual  unfolding  of  earth- 
history — it  is  this,  perhaps  more  than  all  else,  that  fascinates 
the  earnest  student  of  geology.  The  "scientific  imagination," 
therefore,  ought  from  the  first  to  be  stimulated  by  every 
observation  one  makes.  Even  within  the  limits  of  a  single 
quarry  one  may  often  meet  with  evidence  from  which  to 
reconstruct  quite  a  number  of  interesting  geological  episodes. 
Small  and  unimportant  the  phenomena  may  seem  to  be,  but 
the  care  bestowed  on  their  interpretation  will  not  be  lost. 
Gradually,  as  we  continue  our  investigations,  our  eyesight 
becomes  sharpened ;  we  not  only  see  better  than  we  did 
when  we  commenced,  but  are  able  eventually  to  take  a  wider 
outlook,  and  to  piece  together  bits  of  evidence  which  at  first 
might  have  appeared  isolated  and  unconnected.  From  all 
which  it  is  obvious  that  the  observer  who  cultivates  the 
scientific  imagination  is  likely  to  produce  a  better  and  more 
reliable  geological  map  than  the  cartographer  who  declines 
to  look  beyond  the  obvious  facts.  The  former  is  on  the  way 
to  become  a  shrewd  generaliser  and  discoverer ;  the  most  the 
latter  can  hope  to  do  is  to  provide  materials  for  others  with  a 
wider  outlook  to  work  up  and  interpret. 

Geological  Sections. — When  the  geologist  has  completed 
his  map,  he  usually  prepares  one  or  more  horizontal  or  profile 
sections  to  illustrate  the  general  structure  of  the  region.  With 
an  accurately  constructed  map,  sections  might  often  be  dis- 
pensed with,  since  anyone  who  can  read  such  a  map  could 
draw  sections  across  it  in  any  direction.  Few  maps,  however, 
are  large  enough  to  show  all  the  needful  data,  and  the  smaller 
and  more  generalised  the  map  is,  the  more  necessary  do 
explanatory  sections  become.  Two  kinds  of  sections  are  con- 


GEOLOGICAL  MAPS  AND  SECTIONS 


325 


structed,  namely,  Horizontal  or  Profile  and  Vertical^  the 
former  being  designed  to  show  the  form  of  the  ground  and 
the  geological  structure  of  the  region  traversed,  while  the 
latter  are  meant  to  exhibit  in  detail  merely  the  vertical 
succession  of  the  strata. 

Horizontal  (Profile]  Sections. — If  these  are  to  be  accurately 
constructed,  they  must  be  drawn  upon  a  true  scale — that  is, 
the  vertical  and  horizontal  scales  must  be  the  same.  The 
young  geologist's  first  attempts  at  section  drawing  should 
therefore  be  on  this  true  or  natural  scale.  If  the  vertical 
scale  be  exaggerated,  it  is  obvious  that  the  lines  which  are 
meant  to  show  the  geological  structure  must  be  correspondingly 
distorted.  A  glance  at  the  two  sections  (Figs.  122,  123)  will 


FIG.  122. — SECTION  ON  A  TRUE  SCALE — THE  HORIZONTAL  AND  VERTICAL 
SCALES  BEING  THE  SAME. 

serve  to  make  this  plain.  Fig.  122  is  drawn  on  a  natural 
scale,  and  therefore  exhibits  the  actual  form  of  the  surface, 
and  the  true  dip  of  the  strata.  In  Fig.  123  we  have  the 
same  section,  but  in  this  the  vertical  is  three  times  greater 


FIG.  123. — SECTION    ACROSS   SAME   AREA  AS   IN   FIG.   122 — THE  VERTICAL 
BEING  THREE  TIMES  GREATER  THAN  THE  HORIZONTAL  SCALE. 

than  the  horizontal  scale — the  result  being  that  not  only  are 
the  surface  features  grossly  exaggerated,  but  the  geological 
structure  is  distorted — the  inclination  of  the  strata  being 
greatly  in  excess  of  the  true  angle  of  dip.  It  is  only  by 
carefully  plotting  our  sections  to  exhibit  the  actual  form  of 


326  STRUCTURAL  AND  FIELD  GEOLOGY 

the  ground,  that  we  learn  to  appreciate  the  intimate  relation 
that  obtains  between  surface  features  and  geological  structure. 
Everyone  is  prone  to  exaggerate  slopes — even  experienced 
artists  frequently  do  so,  especially  in  the  case  of  mountains — 
and  the  young  geologist  who  neglects  to  train  his  eye  by 
frequent  section  drawing  on  a  true  scale,  is  not  likely  to 
escape  this  common  failing.  The  beginner  will  find  it 
excellent  practice  to  draw  topographical  (not  geological) 
sections  in  all  directions  across  some  of  the  hilly  tracts 
represented  on  the  large  6-inch  maps  of  Scotland.  He  will 
doubtless  be  surprised  to  see  how  inconspicuous  many  heights 
appear  when  drawn  to  scale,  how  relatively  gentle  are  the 
undulations  of  the  surface  even  in  a  mountainous  tract.  In 
the  same  way  he  will  recognise  that  the  deep  basins  occupied 
by  our  larger  lakes  when  seen  in  their  true  proportions  are 
mere  shallow  pans  or  troughs.  Loch  Ness,  for  example,  is 
780  feet  deep,  but  then  it  is  not  less  than  22 J  miles  long,  so 
that  the  length  is  152  times  greater  than  the  depth. 

Of  course  it  is  not  always  possible  to  plot  geological 
sections  on  a  true  scale.  If  the  region  to  be  illustrated  be  of 
great  extent,  say  100  miles  across,  it  is  obvious  that  we  must 
generalise  both  the  topography  and  the  geology.  Even  in 
such  cases,  however,  it  is  important  to  indicate  as  clearly  as 
may  be  the  leading  surface  features  of  the  region  and  the 
relation  of  these  to  the  geological  structure.  A  similar 
remark  holds  good  with  regard  to  all  sketch-sections.  Should 
the  heights  and  slopes  of  the  land  be  so  inconspicuous  as  to  be 
barely  perceptible  when  drawn  upon  a  natural  scale,  it  is  often 
necessary  to  exaggerate  them  in  order  to  show  their  relation 
to  the  internal  structure.  The  exaggeration,  however,  should 
not  be  so  pronounced  as  to  amount  to  positive  distortion. 

In  running  a  geological  section,  care  should  be  taken  to 
draw  it  as  nearly  as  possible  at  right  angles  to  the  strike.  If 
the  strata  be  inclined  in  the  same  direction  throughout  a 
district,  the  section  will  necessarily  follow  a  straight  line. 
But  should  the  strike  vary  from  point  to  point,  the  line  of 
section  will  be  correspondingly  sinuous  or  zig-zag.  Reference 
to  Plate  LVI.  will  show  how  frequently  the  direction  of  a 
section-line  must  change,  when  it  is  desired  to  bring  into  one 
connected  view  the  general  geological  structure  of  a  whole 


GEOLOGICAL  MAPS  AND  SECTIONS  327 

district.  The  section  in  question  is  a  mere  diagram,  and  is 
therefore  not  drawn  to  scale,  but  it  exhibits  the  leading 
surface  features  and  their  relation  to  the  underground  struc- 
ture. 

There  is  no  difficulty  in  plotting  a  profile  section  on  a  true 
scale.  If  the  student  has  laid  down  his  geological  lines  upon 
the  6-inch  map  of  the  Ordnance  Survey,  all  he  has  to  do  is 
first  to  draw  a  line  across  the  map  in  the  direction  to  be 
followed  by  his  section.  Next,  on  a  separate  sheet  of  paper, 
he  draws  a  line  to  represent  the  sea-level.  Upon  this  datum- 
line  he  erects  verticals  for  the  heights  of  the  land  traversed 
by  his  section,  which  he  obtains,  of  course,  from  the  contours 
on  the  map.  When  the  extremities  of  these  lines  are  con- 
nected they  give  the  average  form  of  the  ground.  If  it  be 
desirable  to  reproduce  the  surface  features  in  greater  detail, 
the  observer  may  walk  over  the  ground  with  his  section  in 
his  hand,  and  so  modify  the  line  as  to  show  the  subordinate 
irregularities  that  appear  between  the  measured  contours. 
Usually,  however,  this  refinement  is  not  necessary,  when 
the  contour  lines  upon  the  map  succeed  each  other  at 
intervals  of  100  feet  or  less.  At  the  higher  elevations  of  the 
land  the  intervals  between  the  contour-lines  increase  to  as 
much  as  250  ft,  and  when  such  is  the  case  the  geologist  will 
probably  consider  it  advisable  to  revisit  the  ground,  in  order 
to  make  the  upper  line  of  his  section  represent,  as  closely  as 
may  be,  the  varying  configuration  of  the  surface. 

Having  satisfied  himself  as  to  the  correctness  of  this 
upper  line,  he  then  proceeds  to  insert  the  dips  of  the  strata, 
and  every  detail  shown  upon  his  map  along  the  line  of  section. 
Probably  the  section  will  now  and  again  traverse  places 
where  outcrops  are  not  seen,  but,  the  structure  of  the  ground 
having  been  carefully  worked  out,  he  will  usually  have  no 
difficulty  in  filling  in  these  blanks  from  the  evidence  supplied 
by  rock-exposures  seen  elsewhere  on  the  same  geological 
horizon.  After  all  the  data  referred  to  have  been  inserted, 
the  question  arises — how  far  are  the  dips  exposed  at  the 
surface  to  be  continued  downwards?  The  depth  to  which 
we  may  carry  our  lines  will  naturally  depend  upon  the 
geological  structure.  If  our  section  should  traverse  normal 
anticlines  and  synclines>  we  need,  have  no  doubt,  as  to  the 


328 


STRUCTURAL  AND  FIELD  GEOLOGY 


conduct  of  the  lines  below  the  surface.  In  the  case  shown  in 
Fig.  124  the  strata  between  a  and  b  are  obviously  sym- 
metrically folded.  We,  therefore,  should  be  justified  in 
continuing  the  dips  of  the  synclinal  strata  downwards  for 
some  distance  at  the  same  angle  as  they  show  at  the  surface, 
and  then  gradually  cause  the  degree  of  inclination  to  diminish 
until  the  beds  should  become  horizontal  in  the  core  of  the 
synclinal  trough.  If  the  rock-folds,  instead  of  being  "open" 
as  shown  in  the  diagram  at  a,  were  closely  compressed  as 
between  b  and  ^,  the  curves  of  synclinal  and  anticlinal  cores 
would  be  more  or  less  sharply  angular ;  and  in  our  section  we 
should  have  to  continue  the  limbs  of  a  syncline  downwards 


FIG.  124. — DIAGRAM-SECTION. 

Vertical  lines = heights  above  datum-line  (d).    Continuous  lines  =  outcrops.    Interrupted  and 
dotted  lines  =  inferred  direction  of  strata  below  the  surface. 

for  a  relatively  greater  distance  before  the  trough  core  was 
reached.  In  short,  when  gently  inclined  strata  dip  towards 
each  other  at  approximately  the  same  angle,  we  may  be  sure 
that  the  inclination  of  the  limbs  will  rapidly  lessen  as  they 
approach  the  trough  core,  while  in  the  case  of  steeply  inclined 
and  closely  compressed  unsymmetrical  folds,  the  limbs  of  a 
syncline  must  descend  to  a  relatively  greater  depth  before 
they  meet. 

In  regions  which  have  not  been  topographically  sur- 
veyed, or  the  maps  of  which,  if  such  exist,  give  very  few 
elevations,  the  geologist  must,  of  course,  do  his  own  level- 
ling, if  he  desires  to  plot  a  section  on  a  true  scale.  In  such 
a  case  he  may  select  any  line  for  the  base  of  his  section — 
either  the  sea-level,  the  surface  of  some  lake,  or  the  bottom 
pf  some  valley,  etc.,  or  he  may  prefer  to  erect  his  section 


GEOLOGICAL  MAPS  AND  SECTIONS  329 

on   some   imaginary  line  drawn   at    any  distance  below  the 
surface. 

Vertical  Sections. — These  are  sections  so  drawn  as  to 
show  all  the  strata  piled  up,  as  it  were,  in  a  tall  column  in 
their  proper  order  of  succession.  Where  no  unconformities 
occur,  the  dip  of  the  strata  is  usually  neglected,  and  the  beds 
are  arranged  in  a  horizontal  position.  As  sections  of  this 
kind  are  meant  to  show  in  detail  the  succession  of  the  strata 
in  a  coal-field  or  other  area  containing  beds  and  seams  of 
economic  importance,  they  are  drawn  on  a  large  scale — a 
much  larger  scale  than  would  be  employed  in  the  construction 
of  even  the  most  elaborate  profile  section.  In  the  case  of 
vertical  sections  great  accuracy  is  required — the  thickness  of 
each  individual  bed  being  carefully  measured  in  exposed 
rock-sections,  or  obtained  from  other  reliable  sources,  as  from 
records  of  the  rocks  passed  through  in  sinking  wells,  pits, 
bore-holes,  etc.  The  Geological  Survey  publishes  sheets  of 
such  sections  to  show  the  succession  of  beds  encountered  in 
our  coal-fields.  By  comparing  the  vertical  sections  illustrating 
any  particular  coal-field,  we  can  see  at  a  glance  how  the  same 
series  of  strata  varies  as  it  passes  from  one  part  of  the  coal- 
field to  another.  Similar  vertical  sections,  usually  on  a  much 
smaller  scale,  are  now  and  again  constructed  by  geologists 
for  the  purpose  of  comparing  the  succession  of  strata  met 
with  in  one  place  with  that  encountered  in  some  other  area 
where  rocks  of  the  same  age  occur.  It  is,  in  short,  a  graphic 
method  of  showing  how  the  same  formations  and  systems 
vary  in  character  as  they  pass  from  one  region  to  another. 


CHAPTER   XXII 

ECONOMIC  ASPECTS   OF   GEOLOGICAL   STRUCTURE 

The  Search  for  Coal — Conditions  under  which  Coal  occurs.  Trial 
Borings.  The  Search  for  Ores  —  General  Considerations  which 
should  guide  the  Prospector  ;  Nature  of  the  Evidence.  Geological 
Structure  and  Engineering  Operations  —  Excavations,  Tunnels, 
Foundations. 

IN  preceding  chapters  dealing  with  Tectonic  or  Structural 
Geology,  much  that  is  of  interest  and  importance  to  engineers 
and  others  has  been  set  forth  in  more  or  less  detail.  No 
attempt,  however,  has  been  made  to  indicate  the  various 
ways  in  which  a  knowledge  of  rock-structures  may  be  utilised 
by  mining  and  civil  engineers,  architects,  and  others — for 
the  simple  reason  that  the  application  of  the  knowledge  in 
question  must  be  sufficiently  evident.  In  the  present  chapter, 
however,  it  may  not  be  out  of  place  to  give  a  few  supple- 
mentary notes  which  could  not  be  well  inserted  in  earlier 
pages. 

The  Search  for  Coal. — In  regions  the  geological  structure 
of  which  is  well  known,  and  good  maps  of  which  are  available, 
not  much  difficulty  need  be  experienced  by  the  mining 
engineer  who  can  read  and  interpret  geological  maps  and 
sections.  He  may  often  be  at  a  loss,  however,  in  searching 
for  coal,  etc.,  in  a  country  the  geology  of  which  is  only 
imperfectly  understood,  or  even  not  known.  Under  such 
conditions  his  first  care  would  necessarily  be  to  ascertain 
the  geological  age  of  the  sedimentary  strata  by  searching 
for  fossils.  Coal  occurs  in  several  geological  systems.  In 
Britain,  workable  seams  are  practically  confined  to  the 
Carboniferous  system,  and  the  same  is  largely  the  case  in 
many  other  regions^  both  in  Europe  and  North  America, 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE  331 

In  Australia,  India,  and  Southern  Africa,  and  in  Virginia 
and  North  Carolina,  U.S.A.,  workable  coal  occurs  on  higher 
geological  horizons — namely,  in  late  Palaeozoic  and  early 
Mesozoic  strata.  Here  and  there,  but  only  at  wide  intervals, 
seams  of  economic  value  are  also  encountered  in  the  younger 
Mesozoic  systems  (Jurassic  and  Cretaceous).  The  Cainozoic 
strata  in  several  regions  yield  lignite  or  brown  coal,  as  in 
North  Germany,  Italy,  and  Washington,  U.S.A.  The  mining 
engineer  who  may  be  on  the  outlook  for  coal  will  do  his 
best,  therefore,  to  discover  fossils — the  presence  of  which 
will  determine  the  geological  horizon  of  the  strata.  Should 
the  fossils  prove  the  strata  to  belong  to  an  older  period 
than  the  Carboniferous,  workable  coals  are  not  likely  to  be 
met  with.  Hopes,  however,  may  be  encouraged  should  the 
rocks  prove  to  be  of  Carboniferous  or  later  age.  In  such  a 
case  the  engineer  would  endeavour  to  ascertain  the  general 
character  of  the  strata.  Should  thick  marine  limestones  be 
present  in  the  series,  and  the  accompanying  shales  and 
sandstones  yield  only  brachiopods,  cephalopods,  and  other 
types  of  marine  life,  and  should  few  or  no  traces  of  land- 
plants  occur,  then  there  would  be  little  probability  of  dis- 
covering workable  coal-seams.  Should  the  strata,  on  the 
other  hand,  consist  of  rapidly  alternating  beds  of  sandstone 
and  black  or  dark  coloured  shale  with  occasional  seams  of 
clay,  and  layers  of  clay-ironstone — such  a  succession  might 
be  considered  hopeful.  The  appearance  here  and  there  of 
thin  limestones  of  marine  origin  would  not  necessarily  be 
an  adverse  sign,  for  coals  and  limestones  are  well  known  to 
occur,  now  and  again,  in  one  and  the  same  group  of  strata. 
But  should  the  constantly  recurring  beds  of  shale  contain 
plant-remains,  often  very  well  preserved,  while  the  sand- 
stones showed  streaks  and  thin  lenticular  layers  of  coaly 
matter,  and  the  clays  were  charged  with  rootlets,  our  hopes 
of  encountering  coal  would  be  greatly  increased.  It  must 
be  remembered,  however,  that  the  occurrence  of  black  shales 
is  not  of  itself  a  sure  indication  of  the  presence  of  coal  in 
any  series  of  strata.  In  almost  all  geological  systems,  particu- 
larly in  some  of  the  older  Palaeozoic  formations,  black  shales 
may  occur  without  the  accompaniment  of  even  the  most 
exiguous  seam  of  coal.  Let  us  suppose,,  however*  that  our 


332  STRUCTURAL  AND  FIELD  GEOLOGY 

engineer  has  encountered  a  succession  of  strata  closely 
resembling  those  which  in  other  parts  of  the  world  have 
yielded  coal-seams.  He  naturally  examines  carefully  every 
rock-section  in  the  hope  of  discovering  outcrops  of  the  fossil 
fuel.  This  hope,  however,  might  not  be  realised,  for,  owing 
to  weathering,  sections  are  often  rendered  more  or  less 
obscure,  and  outcrops  of  readily  yielding  beds  are  frequently 
concealed  under  sheets  of  debris.  Search  in  the  alluvial 
deposits  of  the  valleys,  however,  might  be  rewarded  by  the 
discovery  of  fragments  of  coal,  and  the  source  of  these  would 
be  tracked  up-stream  in  the  usual  way  (see  p.  289).  Were 
the  region  not  thickly  covered  with  superficial  accumulations, 
outcrops  of  coal  might  be  expected  to  betray  their  presence 
by  the  blackened  soils  and  subsoils  resting  upon  them. 

Having  satisfied  himself  as  to  the  presence  of  coal,  the 
engineer  would  proceed  to  open  up  any  outcrop,  so  as  to 
ascertain  the  thickness  and  quality  of  the  seam,  and  the 
nature  of  the  "roof"  and  "floor."  Should  the  coal  be  too 
thin  or  too  poor  in  quality  to  pay  the  cost  of  working,  we 
should  not  necessarily  give  up  all  hope.  The  engineer  would 
probably  suggest  that  before  abandoning  the  search  the 
ground  should  be  proved  by  putting  down  one  or  more 
bore-holes.  Before  any  such  attempt  is  made,  however,  the 
geological  structure  of  the  area  ought  to  be  worked  out,  the 
character  of  the  strata  most  carefully  noted,  and  some  reason- 
ably clear  notion  formed  as  to  the  conditions  under  which 
the  strata  have  been  accumulated.  It  may  be  that  the  coals 
are  persistent  at  definite  horizons  throughout  the  whole  area, 
or,  on  the  other  hand,  they  may  be  merely  lenticular  seams 
of  no  great  extent,  and  occurring  at  irregular  intervals.  In 
the  former  case,  it  is  obvious  that  a  valuable  coal-field  would 
be  waiting  development,  while  in  the  latter  case  the  chances 
of  striking  a  workable  seam  might  be  too  uncertain  to  attract 
the  attention  of  capitalists. 

Careful  investigation  of  the  exposed  sections  should 
enable  the  observer  to  decide  the  question.  If  he  be  a 
geologist  he  will  know  that  coal  has  been  formed  in  two  ways  : 
sometimes  it  represents  swampy  accumulations — the  accumu- 
lated growth  of  plants  in  situ — at  other  times  it  would  appear 
to  consist  of  drifted  vegetable  debris,  Jn  either  case  the  fossil 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE  333 

fuel  has  resulted  from  the  chemical  alteration  of  vegetable 
matter  while  excluded  from  the  action  of  the  atmosphere. 
Coals  vary  much  in  character — even  individual  seams  vary 
considerably,  being  more  or  less  bituminous  as  the  case 
may  be.  These  differences  are  doubtless  largely  due  to  the 
nature  of  the  original  vegetable  debris.  Certain  parts  of  the 
plants  of  Carboniferous  times,  for  example,  were  more  resinous 
than  others,  and  when  such  enter  largely  into  the  formation  of 
a  seam  we  have  usually  a  highly  bituminous  coal.  The  cones 
and  spikes  of  the  vascular  cryptogams  and  the  gymnosperms 
shed  abundance  of  resinous  spores  and  pollen,  while  some  of 
the  old  coal-trees  seem  to  have  secreted  resin — the  cortical 
substance  of  certain  types  being  traversed  by  tubes  and  canals 
which  are  believed  to  have  been  connected  with  this  function. 
Leaves,  woody  matter,  and  bark  constitute  the  major  portion 
of  most  mineralised  coal. 

The  two  views  at  present  maintained  as  to  the  mode  of  formation  of 
coal-seams  may  be  very  shortly  described. 

(a)  Growth  in  Situ. — This  is  thought  to  be  the  probable  origin  of 
coal-seams  that  retain  a  tolerably  uniform  thickness  over  extensive  areas, 
and  which  rest  on  clay  or  shale  containing  abundant  rootlets.     Seams  of 
this  kind  are  supposed  to  have  been  formed  much  in  the  same  way  as 
mangrove-    and    cypress-swamps.      The    vegetation    may    have    been 
developed  partly  on  the  low  flat  shores  of  estuaries  and  bays,  and  partly 
in  the  salt  water  itself,  for  marine  shells  now  and  again  are  found  closely 
associated  with  such  coal-seams.     Many  successive  beds  of  coal  may 
occur  in  a  very  thick  consecutive  series  of  sandstones,  shales,  clays,  etc. — 
all  these  beds  having  been  laid  down  in  relatively  shallow  water.     Such 
a  succession  would  indicate  accumulation  during  a  prolonged  period  of 
subsidence — interrupted,  perhaps,  by  longer  or  shorter  pauses..  Each 
coal  with -its  underclay  would  in  this  view  represent  such  a  pause  in 
the  movement  of  subsidence. 

(b)  Drifted    Vegetable  Debris.  —  Some   coals  cannot   possibly   have 
resulted  from  the  growth  of  plants  in  situ.      This  is   proved  by  the 
constant  dovetailing  and  interosculation  of  such  seams  with  shale  and 
sandstone,  and  by  the  very  irregular  thickness  of  the  coals  themselves. 
Well-preserved  ferns,  leaves,  branches,  etc.,  may  abound  in  the  beds 
immediately  underlying,  and  particularly  in  those  overlying  such  coals. 
Now  and  again  stems  of  trees  in  approximately  upright  positions  occur 
in  the  associated  sandstones.     Their  appearance  is  suggestive  of  flotage 
— for  a   long  bare   stem,   crowned  atop  with    an   abundance   of  leafy 
branches,  and  weighted  below  with  its  thick  but-end  and  roots,  would 
tend  to  sink  in  a  more  or  less  upright  position,  swaying  over,  perhaps, 
in  the  direction  followed  by  the  transporting  current. 


334  STRUCTURAL  AND  FIELD  GEOLOGY 

The  occurrence  of  numerous  partings  of  shale  in  certain  coal-seams, 
and  the  fact  that  such  partings  thicken-out  in  many  cases  so  as  eventu- 
ally to  separate  one  coal-seam  into  two  or  more  seams,  divided  the  one 
from  the  other  by  many  feet  or  yards  of  sedimentary  strata — are  hardly 
consistent  with  the  theory  of  growth  in  situ.  Nor  does  this  theory 
account  satisfactorily  for  the  well-known  fact  that  individual  coal-seams 
often  consist  of  different  kinds  of  coal,  occurring  one  above  the  other  in 
irregular  lenticular  layers,  or  in  more  or  less  persistent  seams.  For 
example,  many  common  coals  contain  interlaminations  of  cannel  coal, 
which  may  be  an  inch  or  two  in  thickness,  while  some  layers  of  certain 
common  coals  are  more  bituminous  than  others.  Further,  it  may  be 
noted  that  fish-teeth,  and  shells  of  what  were  either  fresh-water  or 
brackish-water  molluscs,  now  and  again  occur  in  coals  or  in  the  shales 
immediately  associated  with  them  ;  even  marine  shells,  as  we  have  seen, 
occasionally  appear  in  connection  with  coals.  In  the  Scottish  coal-fields, 
indeed,  thin  coals  not  infrequently  are  underlaid  or  overlaid  directly  with 
calcareous  shales  and  thin  limestones,  throughout  which  marine  organic 
remains  abound.  These  phenomena  are  very  hard  to  explain  in  any 
other  way  than  by  supposing  that  such  coals  were  accumulated  in  water 
—that,  in  short,  they  are  sedimentary  formations.  It  has  been  objected 
that  the  comparative  purity  of  coal — the  relative  absence  of  sand  and 
mud — is  against  the  supposition  that  coal  could  have  been  accumulated 
by  drift.  It  is  obvious,  however,  that  vegetable  matter  carried  down  by 
streams  and  rivers  into  lakes  and  estuaries  does  not  necessarily  become 
impregnated  or  mixed  up  with  ordinary  sediment.  When  a  river  enters 
an  estuary,  its  sediments  become  sifted  out — the  finer  ingredients  being 
spread  over  the  widest  area.  Should  the  river  carry  rafts  and  sheets  of 
vegetable  debris,  these  may  well  be  transported  far  beyond  the  reach  of 
ordinary  sediment.  Becoming  waterlogged,  this  vegetable  debris  would 
eventually  sink,  and  might  therefore  come  to  rest  in  regions  rarely  or 
never  reached  by  ordinary  sedimentary  matter.  It  is  quite  conceivable, 
therefore,  that  over  the  floors  of  estuaries  and  broad  bays,  at  some 
distance  from  the  land,  vast  accumulations  of  vegetable  detritus  might 
take  place.  The  interosculation  of  such  sheets  of  vegetable  matter  with 
sand  and  clay  is  just  what  one  would  expect  to  occur,  while  the  occasional 
appearance  of  fresh  water,  estuarine,  or  marine  organisms  need  not 
surprise  us,  for  the  accumulation  of  vegetable  debris  might  take  place 
either  in  large  rivers,  in  lakes,  in  estuaries,  or  in  more  open  bays  of  the 
sea.  It  is  obvious,  moreover,  that  the  drift  hypothesis  offers  a  feasible 
explanation  of  the  variable  character  of  many  coals,  and  accounts  well 
enough  for  the  frequent  intercalation  in  the  sandstones  of  thin  lenticular 
layers  and  beds  of  coal  and  coaly  matter. 

Although  the  observer  may  have  assured  himself  that 
only  thin  coals  crop  out  at  the  surface,  he  need  not,  on  that 
account,  conclude  that  further  research  is  useless.  It  is  quite 
possible  that  the  seams  seen  in  actual  section  may  thicken- 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE    335 

out  as  they  are  followed  downwards.  But  if  the  strata  are 
undulating  and  the  same  beds  come  again  and  again  to  the 
surface — each  recurring  coal-seam  continuing  thin  and  unim- 
portant— he  will  have  good  reason  to  infer  that  the  series  is 
not  worth  further  investigation.  Should  the  rocks,  however, 
be  partially  concealed  by  overlaps  or  unconformities  or  by 
faulting,  or  otherwise  not  be  accessible,  owing,  perhaps,  to 
thick  coverings  of  surface  accumulations  or  to  the  paucity  of 
sections,  the  observer  would  not  be  wise  to  abandon  his 
search.  The  occurrence  of  thin  seams  of  coal,  or  even  the 
mere  presence  of  numerous  plant-remains  such  as  are 
commonly  associated  with  coal-bearing  strata,  taken  in  con- 
nection with  the  obviously  shallow-water  origin  of  the  strata 
and  the  absence  of  any  evidence  of  deep-sea  or  purely  marine 
conditions,  would  be  sufficient  to  justify  trial-boring.  The 
frequent  occurrence  of  rootlet-beds,  with  or  without  over- 
lying coal-seams,  would  suggest  the  probability  that  the 
unexplored  parts  of  the  series  might  contain  more  or  less 
persistent  beds  of  coal.  The  absence  of  rootlet-beds,  however, 
would  not  be  quite  so  favourable  a  sign.  It  would  lead  to  the 
inference  that  any  workable  coal-seams  that  might  occur  would 
be  apt  to  be  somewhat  inconstant — lenticular  sheets,  thickening 
and  thinning  irregularly.  But,  inasmuch  as  seams  of  this 
character  not  infrequently  attain  a  very  considerable  thickness 
and  extend  over  wide  areas,  it  would  obviously  be  important 
to  ascertain  the  direction  in  which  thickening  was  likely  to 
take  place,  and  thereafter  to  test  the  ground  by  borings. 

The  Search  for  Ore-Formations. — Bedded  ores  occur 
under  the  same  conditions  as  any  other  sedimentary  rock,  and 
are  therefore  to  be  sought  for  and  traced  by  the  ordinary 
methods  employed  in  field  geology.  The  same  to  a  large 
extent  is  true  of  lodes  and  irregular  ore-formations  of  all 
kinds,  but  the  search  for  these  is  not,  as  a  rule,  so  easy. 
Perhaps  most  discoveries  of  such  ore-deposits  have  been  the 
result  of  accident.  A  large  number,  however,  must  be  credited 
to  those  sanguine  and  often  admirable  observers,  known  as 
"  prospectors,"  the  most  successful  of  whom  have,  wittingly  or 
unwittingly,  usually  followed  geological  methods  of  research. 
Having  become  familiar  with  the  ore-formations  of  some 
particular  region,  and  learned  to  recognise  the  manifold 


336  STRUCTURAL  AND  FIELD  GEOLOGY 

appearances  presented  by  them  at  the  surface,  such  as  the 
coloration  of  soils  and  subsoils,  the  character  of  the  gossans, 
etc.,  a  prospector  could  hardly  fail  of  success  so  long  as  his 
researches  were  confined  within  the  same  geological  area. 
Should  the  same  observer,  however,  essay  to  prospect  in  a 
totally  different  region,  he  might  often  be  nonplussed.  In 
point  of  fact,  many  mining  men  have  wasted  time  and 
substance  in  exploring  wide  tracts  of  country  which  a 
knowledge  of  geology  might  have  led  them  to  avoid,  as 
probably  barren  ground ;  while,  on  the  other  hand,  such 
knowledge  might  have  directed  their  attention  to  areas  in 
which  prospecting  was  likely  to  lead  to  successful  results. 

Lodes  and  irregular  ore-formations  may  occur  in  almost 
any  kind  of  rock,  and  are  not  restricted  to  a  particular 
geological  horizon,  for  they  are  met  with  in  Palaeozoic, 
Mesozoic,  and  Cainozoic  rocks  alike.  But  although  this  is 
true,  yet  by  far  the  larger  number  of  such  formations  are 
associated  with  the  older  geological  systems.  It  is  exceptional 
to  meet  with  valuable  lodes,  etc.,  in  Cainozoic  and  even  in 
Mesozoic  rocks,  except  in  the  vicinity  of  eruptive  masses. 
Most  lodes,  etc.,  are  of  more  or  less  deep-seated  origin,  and 
are  thus  least  likely  to  be  met  with  traversing  rocks  of 
relatively  late  geological  age.  They  are,  therefore,  to  be 
sought  for  in  the  more  ancient  rocks,  because  these  have 
experienced  vast  denudation,  their  present  surface  in  many 
cases  being  several  thousand  feet  or  yards  below  that  which 
existed  at  the  time  the  lodes  were  being  filled.  It  must  not 
be  supposed,  however,  that  every  region  of  highly  denuded 
ancient  rocks  is  likely  to  contain  valuable  ore-formations. 
We  know,  in  fact,  that  such  is  not  the  case ;  but  certainly  it  is 
to  such  regions  that  the  prospector  ought  to  turn  his  attention. 
The  more  highly  folded,  fractured,  and  dislocated  the  rocks 
are,  the  better  from  his  point  of  view,  while  the  presence  of 
batholiths,  sills,  and  dykes  of  eruptive  rock  would  be  rather  a 
hopeful  sign,  as  will  be  gathered  from  what  has  been  set  forth 
in  Chapters  XVI.  and  XVII.  as  to  the  mode  of  occurrence 
and  origin  of  ore-formations. 

Let  us  suppose,  therefore,  that  an  explorer  has  entered  a 
sorely  denuded  hilly  or  mountainous  region  of  ancient  rocks — 
the  general  character  and  geological  structure  of  which  seem 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE    33? 

favourable.     It  is  obvious   that   he   must   be   guided    in   his 
investigations  by  the   principles   already   illustrated   in   con- 
nection  with  geological   map-making.     He  must  be  on   the 
outlook  for  every  indication  that  may  lead  him  to  the  hidden 
treasure.     Any  marked  change  in  the    form    of  the   ground 
will  be  noted — such  as  a  prominent  narrow  ridge  running  in 
a  linear  direction — or  a  like  well-marked  hollow.     The  former 
may  mark  the  outcrop  of  a  lode  of  harder  consistency  than 
the  rocks  it  traverses,  while  the  latter  may  indicate  the  back 
of  a   vein   filled  with  less  resistant   mineral   matter.      Even 
when   the   outcrop   of  a   lode   produces   no   marked   surface 
feature,  it  will  yet,  in  many  cases,  betray  its  presence  by  more 
or  less  pronounced   coloration    of  the  soil.     This   is   due,  of 
course,  to  the  decomposition  of  the  minerals  occurring  in  the 
vein.     Very  frequently  the  overlying  soil  will  be  stained  red 
or  yellow,  owing  to  the  presence  of  iron-oxides,  which  are  of 
common    occurrence    in    most   gossans.      Other    minerals,   if 
sufficiently  abundant,  will  indicate  their  presence  by  various 
tints   or   hues.     Green  colours,  for   example,  are  yielded   by 
ores  of  copper,  nickel,  or  chromium  ;  commingled  blue,  green, 
and  red  stainings  are  yielded  by  copper  ores ;  lead-ore  is  often 
indicated    by   yellow   and   green   stainings ;   manganese-ores 
are  black,  while  auriferous  quartz  is  often  rusty  and  cellular 
from  the  removal  of  pyrites.     Again,  a  lode  may  be  indicated 
by  springs   coming   to   the  surface  along  a  definite  line,  for 
lodes  often  act  as  subterranean  dams  ponding  back  the  water 
that  descends  towards  them  along  the  various  division-planes 
of  rocks,  and   forcing   it   to   the   surface.     Such  springs  not 
infrequently  contain   much   mineral   matter  in   solution,  and 
may  give  rise  to  superficial  accumulations  of  tufa,  limonite, 
etc.     Should  the  water  contain  deleterious  ingredients  (derived, 
for  example,  from  the  decomposition  of  iron  pyrite),  it  will 
naturally  produce  a  more  or   less   striking   effect   upon   the 
vegetation.      A   pyritous   lode   is,   in    this    way,   sometimes 
indicated  by  the  poverty-stricken  character  of  the  vegetation 
in    its   immediate   neighbourhood,  which   may  be   in   strong 
contrast  with  that  covering  the  country-rock. 

Fragments  of  veinstone  and  ore  occurring  in  a  water- 
course will  naturally  be  suggestive,  and  should  the  fine  gravel 
and  sand  of  the  stream  contain  grains  and  particles  of  gold 

Y 


338  STRUCTURAL  AND  FIELD  GEOLOGY 

or  other  valuable  mineral,  the  prospector — always  a  sanguine 
man — will  feel  confident  of  success.  Stopping  ever  and  anon, 
as  he  proceeds  up-stream,  to  examine  the  contents  of  the 
alluvial  deposits,  he  may  at  last  reach  a  point  above  which 
no  fragments  or  particles  of  metal  or  ore  can  be  found. 
Searching  the  adjacent  valley  slopes,  the  observer  discovers 
perhaps  scattered  fragments  of  veinstone  and  possibly  ore. 
But  whether  these  do  or  do  not  occur,  the  prospector  would 
certainly  be  justified  in  digging  pits  or  trenches  down  to  the 
solid  rock,  in  hopes  of  striking  the  ore-formation  itself.  While 
the  search  for  such  formations  is,  for  obvious  reasons,  most 
promising  in  valleys,  yet  the  land-surface  separating  one 
valley  from  another — more  especially  if  it  be  a  hilly  region — 
must  not  be  neglected.  For,  owing  to  long-continued 
weathering,  such  surfaces  are  often  sprinkled  in  places  with 
angular  fragments,  or  partly  mantled  by  sheets  of  rock- 
rubbish.  Should  lodes  traverse  such  a  tract,  they  are  almost 
sure  to  be  betrayed,  even  in  the  absence  of  conspicuous 
outcrops,  by  the  presence  of  loose  fragments,  or  shode-stones, 
as  they  are  termed  by  Cornish  miners.  By  the  careful  track- 
ing of  these  stones  their  source  may  be  located.  It  need 
hardly  be  added  that  the  prospector  who  has  a  good  working 
knowledge  of  ores,  and  is  quick  to  understand  leading 
geological  structures,  is  more  likely  to  succeed  than  the 
geologist  who,  however  expert  he  may  be  in  unravelling  and 
interpreting  rock-structure,  is  yet  unfamiliar  with  the  various 
"  indications "  that  reveal  so  much  to  a  keen-eyed  mining 
man.  The  latter,  however,  whose  experience  may  have  been 
gained  in  some  limited  region,  is  often  at  a  disadvantage 
when  he  begins  prospecting  in  a  new  country.  Not  infre- 
quently he  is  possessed  with  the  belief  that  the  mining  region 
in  which  he  was  brought  up  must  be  the  type  of  all  others. 
If  the  only  valuable  lodes  of  that  region  have  a  north  and 
south  direction,  he  expects  that  the  same  is  likely  to  hold 
good  elsewhere — no  matter  what  the  geological  structure  of 
the  ground  may  be.  He  often  makes  similiar  assumptions  as 
to  the  gossans.  The  auriferous  reefs  of  the  country  he  has 
left  may  crop  out  as  ridges  of  ferruginous  cellular  quartz,  and 
when  similar  gossans  are  encountered  in  a  totally  different 
area,  he  is  apt  to  jump  to  the  conclusion  that  these  also  must 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE    339 

be  gold-bearing,  although  not  a  trace  of  gold  may  occur 
either  in  the  gossans  or  the  veins  they  cover.  Experience 
has  shown,  therefore,  that  it  is  never  safe  to  infer  that  the 
appearances  presented  by  the  shode-stones  and  gossans  of 
one  region  must  necessarily  be  characteristic  of  mineral 
regions  elsewhere.  They  may  or  may  not  be,  but  only  by 
careful  examination  of  the  gossans  and  the  unaltered  veins 
below  can  doubt  be  set  aside. 

Although  lodes  and  irregular  ore-formations  occur  more 
commonly  in  association  with  the  older  geological  systems, 
yet  under  certain  conditions  Mesozoic  and  even  Cainozoic 
rocks  have  become  charged  with  valuable  ores.  The  most 
important  ore-bodies  of  this  late  age  are  met  with  in  the 
vicinity  of  massive  igneous  rocks  (andesites  and  rhyolites)  in 
volcanic  regions  which  have  experienced  much  erosion.  The 
ores  in  question  frequently  appear  as  contact-formations,  and 
the  prospector,  therefore,  should  do  his  utmost  to  trace  the 
line  of  junction  between  such  an  intrusive  mass  and  the  rocks 
it  traverses.  Should  the  effects  of  solfataric  action  be 
manifest,  this  will  be  so  far  a  favourable  sign,  since  it  is  just 
under  these  conditions  that  fissures  and  faults  have  in  many 
cases  been  filled  with  ores  and  other  minerals,  and  the 
adjacent  "country-rock"  has  been  impregnated.  Now  and 
again,  however,  the  observer  finds  that  denudation  has  not 
yet  laid  bare  the  intrusive  masses  from  which  heated  solutions 
have  been  derived — they  still  lie  more  or  less  deeply  buried 
underneath  the  strata  which  have  been  affected  by  them. 
Their  presence  is  suggested,  in  the  first  place,  by  the  changes 
which  these  rocks  have  undergone.  The  latter  are  often 
much  disturbed,  shattered  and  brecciated,  and  frequently 
highly  silicified — shales  being  converted  into  hard,  siliceous 
porcellanite,  and  limestones  into  quartzose  marbles,  while 
felspathic  rocks  are  usually  kaolinised.  Moreover,  dykes  of 
quartz-porphyry,  etc.,  are  often  more  or  less  plentifully  present. 
Native  gold  and  various  ores  of  silver,  antimony,  mercury, 
arsenic,  etc.,  frequently  impregnate  and  are  disseminated 
through  the  silicified  rocks  under  such  conditions.  The  great 
volcanic  tracts  bordering  the  Pacific  in  North  and  South 
America,  and  the  corresponding  volcanic  regions  of  Japan 
and  other  islands  on  the  opposite  side  of  that  ocean,  are  in 


340  STRUCTURAL  AND  FIELD  GEOLOGY 

places  notable  for  their  ore-formations,  many  of  which  are 
of  late  Mesozoic  and  Cainozoic  age.  There  can  be  little 
doubt,  however,  that  the  mineral  wealth  of  those  lands  is 
as  yet  very  imperfectly  known,  and  that  many  rich  mining- 
fields  still  remain  to  be  discovered  by  properly  qualified 
prospectors. 

Geological  Structure  and  Engineering  Operations: 
Excavations. — In  the  construction  of  roads,  railways,  and 
canals,  the  nature  of  the  rocks  to  be  excavated  must  be 
ascertained  before  any  estimate  can  be  formed  of  the 
probable  cost  of  an  undertaking.  Where  the  rocks  themselves 
are  not  exposed  to  view,  the  engineer  usually  digs  pits  or 
puts  down  shallow  bore-holes,  and  bases  his  estimate  on  the 
information  thus  obtained.  The  evidence  is  often  quite 
sufficient  for  his  purpose ;  at  other  times,  however,  it  is 
inadequate,  and  may  even  be  misleading.  In  the  case  of 
deep  excavations  and  in  the  driving  of  tunnels,  for  example, 
something  more  than  a  mere  knowledge  of  the  rock  immedi- 
ately underlying  the  subsoil  is  required.  Before  undertakings 
of  that  kind  are  entered  upon,  the  engineer  ought  to  make 
a  thorough  examination  of  the  geological  structure,  for  the 
nature  and  character  of  the  rocks  at  the  surface  may  afford 
no  indication  of  the  nature  of  the  rocks  and  rock-structures 
to  be  encountered  at  a  short  distance  below. 

In  all  such  engineering  operations  it  is  most  important 
to  ascertain  (a)  the  lithological  character  of  the  rocks  to  be 
penetrated,  and  (fr)  the  mode  of  their  arrangement  or,  in 
short,  the  geological  structure.  It  is  obvious  that  the  actual 
cost  of  excavating  will  depend,  in  the  first  place,  on  the 
relative  hardness  of  the  rocks,  and  the  ease  with  which  they 
can  be  extracted,  which  will  often  be  determined  by  the 
nature  of  the  jointing.  The  engineer  has  further  to  consider 
whether  the  rocks,  when  cut  through,  are  likely  to  be  self- 
supporting,  and  of  sufficient  durability  to  withstand  the 
action  of  the  weather.  Here  the  question  of  geological 
structure  comes  in,  for  it  is  obvious  that  a  rock  exposed  in 
vertical  section  may  be  sufficiently  durable  if  it  be  horizontally 
bedded,  but  quite  untrustworthy  if  it  be  inclined  in  the 
wrong  direction.  The  behaviour  of  the  rocks  with  regard 
to  the  circulation  of  underground  water  has  also  to  be  taken 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE    341 

into  account.  A  highly  permeable  rock  exposed  in  section 
may  weep  so  copiously  through  pores  or  joints,  that  it  is 
readily  broken  up  when  exposed  to  the  atmosphere. 

As  it  is  hardly  possible  for  the  engineer  to  consider  the 
lithological  character  of  rocks  apart  from  their  geological 
structure,  we  may  briefly  indicate  how  strata  may  be 
expected  to  behave  in  cuttings  according  as  they  occupy 
horizontal  or  inclined  positions. 

When  homogeneous  firm  rocks  which  show  very  few 
joints  are  horizontally  disposed,  they  may  usually  be  relied 
upon  to  be  self-sustaining,  and  to  stand  with  approximately 
vertical  faces  in  any  open  cutting.  Such  rocks  are  permeable 
only  to  a  limited  extent,  and  throw  out  little  or  no  water. 
Hence,  even  when  they  occur  as  a  series  of  thick  beds 
separated  by  intervening  layers  of  impermeable  shales  or 
clay,  springs  are  of  infrequent  occurrence.  Where  the  rocks 
to  be  cut  through  consist,  on  the  other  hand,  of  a  series  of 
highly  porous  and  jointed  beds  with  intervening  impermeable 
shales,  etc.,  springs  and  seepage  may  be  expected.  Even 
should  water  not  filter  through  into  the  cutting,  it  is  obvious 
that  beds  of  such  varying  character  are  sure  to  weather 
unequally — the  softer  rocks  will  crumble  away,  and  constant 
undermining  of  the  overlying  beds  must  take  place.  Under 
such  conditions,  it  becomes  necessary  to  bench  back  the 
cutting,  or  to  slope  it  until  the  angle  of  repose  is  reached. 
But  where  much  water  is  discharged,  the  engineer  may  be 
compelled  to  mask  the  cutting  with  impervious  masonry, 
apertures  being  left  in  the  wall  here  and  there  to  allow  the 
water  to  escape. 

When  strata  are  excavated  in  the  direction  of  their  dip, 
they  can  usually  be  treated  as  if  they  were  horizontally 
bedded.  The  point  of  most  importance  is  the  nature  of 
the  rocks  themselves.  If  the  beds  are  firm  and  relatively 
impermeable,  they  may  be  expected  to  stand  with  vertical 
or  nearly  vertical  faces.  The  chief  dangers  to  be  guarded 
against  are  the  escape  of  water  and  the  action  of  frost. 

Cuttings  made  in  the  direction  of  the  strike  usually 
require  different  treatment  On  one  side  of  such  an  excava- 
tion the  beds  dip  away  from  the  line  of  cutting,  and  therefore 
occupy  a  strong  position.  Even  should  they  consist  of  a 


342  STRUCTURAL  AND  FIELD  GEOLOGY 

series  of  alternating  porous  and  impermeable  rocks,  they  will 
often  stand  with  a  vertical  face.  The  reason  is  obvious,  for 
any  water  the  porous  beds  may  contain  will  tend  to  escape 
downwards  along  the  planes  of  bedding — it  is  drained  away 
from  the  cutting.  On  the  opposite  side  of  the  excavation, 
the  strata  dip  into  the  cutting  and  therefore  occupy  an 
unstable  position.  The  tendency  is  for  the  truncated  beds 
to  slide  downward  ;  and  should  they  consist  of  alternating 
pervious  and  impervious  beds,  springs  will  come  out,  under- 
mining will  take  place,  and  piecemeal  or  wholesale  collapse 
must  result.  Rocks  occupying  such  a  position  must  be  built 
up,  care  being  taken  to  allow  passage  for  the  percolating 
water. 

Excavations  in  massive  igneous  rocks  will  often  stand 
with  vertical  or  approximately  vertical  faces.  The  character 
of  the  jointing,  however,  has  to  be  carefully  considered,  and 
the  possible  action  of  springs,  in  undermining  and  dislodging 
masses,  and  the  general  effect  of  frost,  must  not  be  over- 
looked. Schistose  rocks,  in  like  manner,  are  often  firm  and 
stable  when  opened  up,  more  especially  if  the  excavation  runs 
in  the  same  direction  as  the  dip  of  the  foliation.  But  when 
the  cutting  traverses  such  rocks  along  the  strike,  they  are  apt 
to  behave  much  in  the  same  way  as  sedimentary  strata — on 
one  side  of  the  excavation  they  may  be  expected  to  stand 
well ;  on  the  opposite  side  slips  and  falls  are  likely  to  take 
place.  Their  stability,  moreover,  is  often  affected  by  the" 
very  irregular  jointing,  and  by  the  variable  character  of  the 
rocks  themselves ;  so  that  while  some  schists  readily  allow 
the  passage  of  water,  others  are  more  or  less  impermeable. 
The  stability  of  this  class  of  rocks  further  depends,  to  some 
extent,  upon  the  nature  of  the  foliation.  Evenly  foliated 
rocks,  which  simulate  ordinary  sedimentary  strata,  may  be 
expected  to  behave  much  in  the  same  way  as  the  latter. 
When  schists  are  much  crumpled  and  contorted,  however, 
the  individual  folia  are  more  securely  locked  together,  and 
slipping  is  much  less  likely  to  take  place,  so  that  such  rocks 
may  often  be  treated  as  if  they  were  highly  jointed  igneous 
masses.  Slates,  it  need  hardly  be  said,  more  closely  resemble 
steeply  bedded  sedimentary  rocks,  such  as  greywackes  and 
shales,  the  stability  of  the  faces  of  a  cutting  depending  upon 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE  343 

the  direction  of  the  planes  of  cleavage.  Should  the  latter 
be  traversed  at  right  angles,  the  rock  on  both  sides  of  the 
cutting  will  stand  with  vertical  faces.  When  an  excavation, 
however,  is  carried  in  the  same  direction  as  the  strike  of 
the  cleavage,  the  rock  will  necessarily  be  more  stable  on  one 
side  than  the  other.  In  a  word,  the  superinduced  cleavage- 
structure  plays  the  part  of  lamination  and  bedding  in  the 
case  of  sedimentary  strata. 

It  is  not  necessary  to  add  more  than  a  word  as  to 
excavations  in  incoherent  and  non-consolidated  rocks.  Rocks 
of  this  kind  will  riot  stand  with  a  vertical  face,  except  in 
rainless  or  all  but  rainless  regions.  Under  ordinary  conditions, 
therefore,  the  engineer,  in  excavating  soft,  incoherent  masses 
or  beds,  has  to  consider  the  slopes  he  must  give  to  the  sides 
of  the  cutting,  for  the  angle  of  repose  varies  directly  as  the 
nature  of  the  materials.  Even  in  such  cases  it  will  usually  be 
found  that  geological  structure  has  its  influence.  Should  the 
incoherent  beds  have  a  dip  and  be  traversed  at  right  angles  to 
their  inclination,  they  will  almost  invariably  stand  better  on 
one  side  of  a  cutting  than  the  other.  Any  water  that  may 
permeate  the  beds  will  tend  to  come  out  rather  on  the  "  weak  " 
than  on  the  "  strong  "  side  ;  slipping  will  be  apt  to  take  place 
from  time  to  time  on  the  former,  but  not  so  readily  on  the 
latter. 

Tunnels. — If  it  be  unwise  on  the  part  of  an  engineer  to 
-undertake  important  excavations  or  open  cuttings  without 
having  previously  examined  the  geological  structure  of  the 
ground,  to  be  traversed,  he  would  be  deserving  of  censure  if, 
before  driving  an  important  tunnel,  he  did  not  first  endeavour 
to  ascertain  every  fact  connected  with  the  rocks  and  their 
arrangement.  In  the  case  of  horizontally  bedded  strata,  not 
much  difficulty  would  arise;  the  tunnel  would  be  driven 
practically  along  the  planes  of  bedding,  and  the  character  of 
the  rock  at  the  two  ends  of  the  proposed  tunnel  could  be 
readily  ascertained,  and  thus  a  reliable  estimate  of  the  cost  of 
the  work  could  be  formed.  But  if  the  strata  were  not 
horizontal,  then  even  the  most  careful  examination  of  the 
rocks  at  either  end  of  the  proposed  tunnel  might  deceive  the 
engineer  who  had  neglected  to  ascertain  the  geological  struc- 
ture. The  annexed  diagram  (Fig.  125)  represents  the 


344 


STRUCTURAL  AND  FIELD  GEOLOGY 


\ 


geological  structure  of  a  hill  which  it  is  proposed  to  tunnel 
along  the  line  a  b.  It  is  obvious  that  shallow  pits  and 
borings  put  down  over  the  surface  can  give  no  indication  of 
the  nature  of  the  rocks  which  the  tunnel  is  likely  to  pierce. 

An  engineer,  finding  that  the  rock 
over  the  top  of  the  hill  and  at  the  two 
extremities  of  the  proposed  tunnel, 
were  all  of  a  reliable  kind,  might 
probably  conclude  that  the  whole  hill 
was  composed  of  like  materials.  If 
the  rock  happened  to  be  of  a  self- 
supporting  nature — one  that  required 
little  or  no  expensive  building — he 
would  frame  his  estimates  of  cost 
accordingly.  A  knowledge  of  the 
actual  structure,  however,  would  have 
shown  him  that  the  self-supporting 
stratum  could  continue  but  a  short 
distance  on  the  level  of  the  proposed 
tunnel,  and  would  then  be  succeeded 
by  friable  shales  requiring  support  all 
the  way  to  near  the  middle  of  the  hill, 
where  highly  porous  and  water-logged 
sandstones  might  be  expected  to  add 
still  further  to  the  difficulties  and  cost 
of  the  undertaking.  The  history  of 
engineering  operations  in  this  and 
other  countries  is  full  of  warnings  as 
to  the  danger  of  driving  tunnels  with- 
out having  first  determined  the  geo- 
logical structure  of  the  ground.  Not 
infrequently,  this  requisite  knowledge 
might  have  enabled  the  engineer  to 
avoid  difficulties  and  greatly  increased 
cost  by  some  slight  deviation  of  the  line 
of  his  tunnel  or  by  modifying  the  gradi- 
ents. Even  in  cases  where  such  deviations  and  modifications 
may  be  impossible,  a  knowledge  of  the  difficulties  lying  before 
him  would  yet  greatly  aid  the  engineer  in  making  his 
estimates, 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE  345 

Foundations. — Engineers  and  architects  are  necessarily 
called  upon  to  consider  the  nature  of  the  foundations  on 
which  it  is  proposed  to  build  heavy  structures.  Trial  holes 
will  show  the  nature  of  the  materials,  but  they  do  not  always 
disclose  the  geological  structure,  and  in  the  case  of  very 
heavy  buildings  it  is  absolutely  essential  that  the  latter  should 
be  carefully  ascertained.  For,  however  solid  and  unyielding 
the  substratum  may  seem  to  be,  calculations  as  to  its  stability 
are  liable  to  error  if  its  relations  to  the  immediately  subjacent 
rocks  be  not  taken  into  account.  For  example,  a  firm 
massive  sandstone  may  be  underlaid  by  some  impervious 
slippery  clay,  which,  should  the  strata  have  a  decided  dip, 
may  yield  to  the  great  pressure  of  a  heavy  superstructure, 
and  cause  the  rock-foundation  to  slide  forward  along  the 
plane  of  bedding.  Unconsolidated  materials  often  make  bad 
foundations,  but  tough,  homogeneous  clay,  if  of  sufficient  thick- 
ness, is  usually  reliable.  Alluvial  or  superficial  deposits  of 
every  kind,  however,  are  as  a  rule  unsatisfactory,  and,  in 
the  case  of  important  structures,  usually  require  special  treat- 
ment, involving  often  costly  excavation,  the  driving  of  strong 
and  closely  set  piles,  and  the  formation  of  an  artificial  founda- 
tion of  concrete.  Although  tough  clays,  such  as  the  boulder- 
clay  of  the  Scottish  lowlands,  usually  form  reliable  foundations, 
they  nevertheless  are  sometimes  untrustworthy.  Not  infre- 
quently they  contain  layers  and  beds  of  gravel  and  sand  that 
carry  water,  which,  when  it  escapes  at  the  surface,  tends  to 
undermine  the  overlying  mass,  and  thus  in  time  causes  the 
ground -to  subside.  Before  any  heavy  building  is  raised  upon 
till,  therefore,  it  is  necessary  to  ascertain  by  means  of  boring 
whether  any  water-bearing  beds  be  present.  The  river- 
valleys  of  Central  Scotland  afford  many  examples  of  the 
relative  instability  of  boulder-clay,  when  that  deposit  rests 
upon  an  inclined  surface  of  rock.  In  the  valley  of  the  Esk, 
for  example,  almost  every  house  and  wall  built  upon  the 
slopes  overlooking  the  valley-bottom  afford  evidence  in  their 
cracked  masonry  of  a  slow  and  interrupted  but  nevertheless 
continuous  slipping  of  the  foundations.  The  cause  is  obvious  : 
the  boulder-clay,  which  has  a  coarse,  rubbly  bottom,  rests 
upon  an  inclined  surface  of  sandstones,  shales,  etc.,  from 
which  water  escapes  and  percolates  through  the  stony  and 


346  STRUCTURAL  AND  FIELD  GEOLOGY 

rubbly  base  of  the  clay.  The  latter  thus  becomes  softened 
and  slippery,  and  from  time  to  time  yields,  and  the  mass 
creeps  downwards.  The  boulder-clay,  under  such  conditions, 
is  in  a  state  of  unstable  equilibrium,  the  risk  of  slipping  increas- 
ing with  the  slope  of  the  surface  on  which  the  clay  reclines. 


CHAPTER   XXIII 

ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE— 
continued 

Water-supply.  Lakes  and  Impounded  Streams.  Reservoirs.  Supply 
from  Rivers.  Underground  Water  —  the  Water-level;  Natural 
Springs  as  illustrating  the  course  followed  by  Subterranean  Water  ; 
Surface  and  Deep-seated  Springs.  Common  Wells  and  Driven 
Wells.  Artesian  Wells.  Considerations  to  be  kept  in  view  in  the 
search  for  an  Artesian  Water-supply.  Drainage.  Distribution  of 
Disease  in  relation  to  Geological  Conditions. 

Water-supply. — Superficial  and  underground  sources  of 
supply  alike  depend  upon  the  amount  of  precipitation  and 
the  physical  aspect  and  geological  conditions  of  a  country. 
But  the  relative  amount  of  water  circulating  above  and  below 
ground  respectively  is  determined  mainly  by  the  character  of 
the  rocks  and  the  mode  of  their  arrangement.  Two  regions, 
for  example,  may  have  the  same  amount  of  rainfall,  and, 
nevertheless,  the  one  may  be  little  better  than  a  dry  desert, 
while  the  other  may  rejoice  in  numerous  streams  and  rivers, 
and  be  conspicuous  for  its  fertility.  There  are  many  lands 
that  consist  of  rocks  so  highly  pervious  that  rain  and  melting 
snow  at  once  descend  below  the  surface,  and  streams  and 
rivers  are  impossible — all  the  drainage  being  conducted 
underground.  On  the  other  hand,  we  may  encounter  else- 
where the  opposite  extreme — namely,  a  country  built  up 
of  impermeable  rocks  which  absorb  so  little  water  that 
practically  the  entire  rainfall  flows  in  superficial  courses  to 
the  sea.  Between  these  two  extremes  there  are  many 
gradations — most  lands  being  composed  partly  of  porous 
and  partly  of  impermeable  rock-formations. 

It  will  be  convenient  to  treat  of  the  water-supply  derived 


348  STRUCTURAL  AND  FIELD  GEOLOGY 

from  superficial  sources  apart  from  that  obtained  from  under- 
ground stores,  although  it  is  obvious  that  much  of  the  water 
that  circulates  in  our  streams  and  rivers  has  come  from 
springs. 

Lakes  and  Impounded  Streams. — The  character  of  the 
water  of  a  lake  naturally  depends  upon  that  of  the  catchment 
area,  for  it  is  needless  to  say  that  the  level  of  the  lake  is 
maintained  by  the  rainfall — in  other  words,  by  the  springs 
and  streams  that  feed  it.  Should  the  rocks  within  the  drainage 
area  be  largely  calcareous  the  water  will  be  hard ;  should 
igneous  and  schistose  rocks  predominate,  the  water  will  be 
moderately  soft.  Other  things  being  equal,  the  deeper  and 
larger  a  lake  is,  the  purer  and  colder  must  the  water  be. 
Large  and  deep  lakes  occupying  mountain  valleys,  like  those 
of  our  own  islands,  where  there  is  little  or  no  cultivation  and 
not  much  chance,  therefore,  of  contamination,  are  always 
desirable  sources  of  water-supply.  But  every  country  is  not 
so  fortunate  in  the  possession  of  large  natural  reservoirs,  and 
even  when  these  exist  they  are  often  so  far  removed  from 
centres  of  population  as  to  be  practically  beyond  reach. 
Under  such  circumstances,  engineers  are  required  to  form 
artificial  lakes  by  impounding  streams. 

Reservoirs. — The  formation  of  reservoirs  is  purely  an 
engineering  operation,  but,  like  many  other  undertakings  of 
the  kind,  it  ought  to  be  conducted  with  a  full  knowledge  of 
the  geological  conditions.  If  we  have  the  choice  of  several 
streams,  it  is  obvious  we  should  select  the  purest.  Those 
which  are  most  likely  to  yield  a  desirable  water-supply  will 
usually  occur  in  sparsely  cultivated  districts  which  are  not 
likely  in  the  future  to  attract  much  population,  such  as  the 
high-lying  pastoral  regions  of  our  own  country.  Before 
selecting  a  stream,  however,  the  character  of  the  rocks  within 
the  drainage-area  should  be  carefully  inspected,  for  the 
purpose  of  ascertaining  whether  these  contain  deleterious 
ingredients  which  might  unduly  affect  the  character  of  the 
water-supply.  Usually,  however,  careful  chemical  analyses  of 
the  water  flowing  in  the  main  stream  at  all  seasons  of  the 
year  will  determine  the  suitability  or  otherwise  of  the  supply. 
A  desirable  stream  having  been  obtained,  the  engineer's  next 
care  is  to  select  a  site  or  sites  for  his  storage  reservoirs.  It 


ECONOMIC  ASPECTS  OF   GEOLOGICAL  STRUCTURE  349 

is  at  this  stage  of  his  work  that  he  will  find  a  knowledge  of 
structural  geology  most  helpful.  He  should  carefully  in- 
vestigate the  rocks  occupying  the  proposed  site,  in  order  to 
satisfy  himself  as  to  their  soundness.  Should  they  be  very 
porous  and  much  shattered  and  jointed,  the  conditions  will 
be  unfavourable,  and  a  better  site,  if  possible,  should  be  sought 
for.  This  will  be  all  the  more  advisable  should  the  strata  in 
question  be  inclined  in  the  same  direction  as  the  valley,  for 
under  such  conditions  leakage  is  almost  certain  to  take  place 
— much  water  escaping  along  the  bedding-planes,  not  to 
reappear  at  the  surface,  perhaps,  till  after  an  underground 
course  of  many  miles.  Should  the  inclination  of  the  strata, 
however,  be  in  the  opposite  direction,  there  is  not  the  same 
danger  of  considerable  loss,  since  any  water  that  finds  its  way 
below  the  surface  may  possibly  be  discharged  again  further 
up  the  valley.  It  is  needless  to  say,  however,  that  no  engineer 
would  think  of  forming  a  reservoir  over  an  area  of  highly 
jointed  and  pervious  rocks,  if  he  could  avoid  doing  so. 
Unfortunately,  the  bottom  of  a  valley  is  frequently  covered 
with  thick  alluvial  deposits,  and  the  engineer,  unless  he  knows 
something  of  geological  structure,  may  not  be  aware  of  the 
nature  and  arrangement  of  the  underlying  rocks — even  after 
he  has  tested  the  ground  by  means  of  boreholes. 

The  selection  of  a  site  for  his  embankment  demands  the 
greatest  care.  Sometimes  there  is  no  difficulty — the  bed  of 
the  valley  may  be  deeply  filled  with  tough,  homogeneous  clay, 
than  which  no  better  foundation  could  be  obtained.  It  is 
always  well,  however,  to  make  sure  by  a  series  of  borings 
that  no  water-bearing  beds  are  present  in  or  underneath  the 
clay,  for  in  either  case  these  would  be  sources  of  danger — 
allowing  of  leakage,  and  thus  threatening  the  stability  of  the 
embankment.  As  foundations  for  an  embankment,  highly 
jointed  rocks  of  any  kind — certain  igneous  rocks,  limestones, 
and  loose  shattery  shales  especially — are  to  be  avoided.  When 
the  engineer  has  no  choice  of  sites,  but  must,  if  at  all  possible, 
build  his  embankment  upon  rocks,  the  character  and  structure 
of  which  are  quite  unfavourable,  the  difficulties  he  must 
encounter  will  add  greatly  to  the  cost  of  the  undertaking. 

Rivers. — Some  towns  and  cities  draw  their  water-supplies 
from  the  rivers  on  which  they  are  situated.  In  certain  cases 


350  STRUCTURAL  AND  FIELD  GEOLOGY 

the  water  is  pumped  from  some  point  above  a  town,  while  in 
other  cases  it  is  drawn  from  the  higher  reaches  of  the  river 
and  brought  in  open  courses  or  in  pipes.  This  source  of 
supply  is  seldom  desirable,  but  not  infrequently  no  other 
source  is  available,  in  which  case  the  only  thing  to  be  done 
is  to  look  well  after  the  filtering,  which  doubtless  minimises 
the  danger  of  pollution,  but  cannot  always  be  implicitly  relied 
upon  to  protect  the  population.  It  is  remarkable,  however, 
how  rapidly  rivers  seem  to  purify  themselves  from  the  pollu- 
tions poured  into  them  by  the  villages  and  towns  upon  their 
banks.  Soon  the  water  begins  to  clear — a  foul-smelling  mud 
settling  upon  the  stones  and  gravel  of  their  beds,  and  gather- 
ing here  and  there  in  extra  quantities  along  their  margins. 
Exposed  to  sunlight  and  the  action  of  the  atmosphere,  the 
various  organic  impurities  become  broken  up — a  process  in 
which  numerous  minute  forms  of  life  play  a  not  unimportant 
part — until  eventually  the  river  may  become  bright  and 
sparkling  as  at  first.  All  such  waters,  however,  are  properly 
held  suspect,  and  ought  never  to  be  used  for  domestic 
purposes  before  they  have  been  carefully  examined  and 
declared  safe. 

Underground  Water. — The  proportion  of  the  rainfall 
that  sinks  into  the  ground  naturally  varies  according  to  the 
character  of  the  underlying  rocks.  But,  whatsoever  the 
nature  of  the  rocks  may  be,  they  are  commonly  charged  with 
water  up  to  a  certain  limit  known  as  the  water-level,  the 
depth  of  which  from  the  surface  is  determined  by  the  amount 
of  rainfall,  the  configuration  of  the  surface,  and  the  geological 
conditions.  In  some  districts,  the  level  in  question  may  be 
reached  at  only  a  few  yards  down ;  in  other  places  it  may 
sink  to  great  depths,  and  it  usually  fluctuates  with  the  rainfall. 
Owing  to  these  several  conditions,  a  constant  underground 
circulation  is  kept  up — gravitation  and  hydrostatic  pressure 
forcing  the  water  through  the  pores  and  fissures  of  the  rocks 
until  it  can  escape  at  the  surface  in  the  form  of  springs.  In 
regions  pomposed  chiefly  of  highly  pervious  rocks  of  great 
thickness,  springs  are  of  infrequent  occurrence,  and  are  apt  to 
appear  only  in  the  deeper  depressions  of  the  land.  But  in 
countries  where  the  rocks  are  of  variable  character  and 
structure,  underground  water  may  be  discharged  at  many 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE  351 

different  levels,  in  mountain  regions  and  low  grounds  alike ; 
not  infrequently,  indeed,  copious  springs  of  fresh  water 
coming  from  such  regions  issue  on  the  floor  of  the  sea.  The 
various  divisional  planes  of  rocks — joints,  bedding-planes, 
faults,  etc.,  naturally  constitute  the  chief  underground  water- 
ways. In  the  case  of  soluble  rocks  these  water-ways  become 
widened  by  the  chemical  and  mechanical  action  of  the 
running  water,  until  very  considerable  tunnels  may  be 
worked  out — giving  passage  to  torrents,  streams,  and  rivers. 
Relatively  insoluble  rocks  are  not,  of  course,  traversed  by 
subterranean  channels  of  this  kind,  but,  if  sufficiently  porous, 
they  frequently  contain  enormous  stores  of  water.  When 
such  beds  are  inclined  and  underlaid  by  impermeable  strata, 
the  water  they  contain  naturally  makes  its  way  downward  in 
the  direction  of  dip.  Should  the  strata  be  horizontal,  under- 
ground flowage  nevertheless  does  not  cease — the  water  under 
hydrostatic  pressure  being  forced  to  percolate  through  the 
rocks.  Such  movements,  indeed,  necessarily  take  place 
even  in  amorphous  rocks  which  are  neither  bedded  nor 
jointed. 

In    the    following    case    (Fig.     126)    we    have,    say,    an 
amorphous   mass   of   sand    and   gravel   (a)   resting   upon   a 


FIG.  126. — HEAPING  UP  OF  WATER  IN  SUPERFICIAL  DEPOSITS. 

horizontal  surface  of  impervious  clay  (£).  Rain  falling  on 
the  surface  of  a  is  greedily  absorbed,  and  gradually  sinks 
until  the  bed  becomes  saturated  up  to  a  certain  limit  (ze>),  when 
the  frictional  resistance  to  its  passage  outwards  is  overcome 
by  the  hydrostatic  pressure.  It  is  then  forced  to  flow  along 
the  surface  of  the  underlying  clay,  and  escapes  to  the  light  of 
day  at  s  s  as  a  line  of  seepage  marking  the  junction  between 
the  porous  and  impervious  beds ;  but  should  there  be  irregu- 
larities in  the  surface  of  the  clay,  it  may  issue  in  the  form  of 
definite  springs.  Springs  of  this  kind  are  of  common  occur- 


352 


STRUCTURAL  AND  FIELD  GEOLOGY 


rence  in  horizontally  bedded  rocks  of  varying  character,  some 
being  pervious,  others  impervious,  for  every  porous  stratum 
is  likely  to  contain  a  store  of  water  which  will  ooze  or  flow  out 
wherever  the  beds  are  truncated,  as  on  hill-slopes  and  in 
valleys  (Fig.  127). 

Generally,  however,  strata  are  more  frequently  inclined 
than  horizontal,  and   through   these  water  flows   under  the 


FIG.  127. — DRAINAGE  IN  HORIZONTAL  STRATA. 

combined  influence  of  gravitation  and  hydrostatic  pressure. 
When  such  strata  are  traversed  by  a  valley  running  in  the 
direction  of  strike,  underground  water  tends  to  be  discharged 
only  on  one  side,  namely,  on  that  side  from  which  the 
truncated  beds  dip  into  the  valley.  If  the  beds  dip  at  a 
high  angle,  the  springs  will  usually  be  insignificant,  since 
with  a  high  dip  the  outcrops  will  be  narrow ;  with  a  low  dip, 
on  the  other  hand,  the  discharge  will  be  proportionately 
greater,  for  the  simple  reason  that  the  outcrops  of  the  porous 
beds  will  spread  over  a  wider  area,  and  be  thus  capable  of 
imbibing  a  larger  proportion  of  the  rainfall. 

Synclinal   valleys   are   of  rare   occurrence,   especially   in 
regions  which  have  experienced  much  denudation  ;  but  when 


FIG.  128. — DRAINAGE  IN  SYNCLINAL  STRATA. 

they  do  occur  amongst  water-bearing  and  impervious  strata, 
springs  may  abound  on  both  sides  of  a  valley  (Fig.  128). 
When  a  valley  coincides  with  an  anticline,  however,  the 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STKUCTUftE   353 

geological  structure  is  obviously  quite  unfavourable  to  the 
outflow  of  underground  water,  the  water,  as  in  the  previous 
cases,  making  its  way  in  the  direction  of  dip  (Fig.  129),  and 
therefore  away  from  the  valley. 

We  have  been  considering  the  flow  of  water  through 
porous  rocks  as  being  conducted  along  the  planes  of  bedding, 
but  we  must  not  forget  that  sedimentary  strata  are  traversed 
by  joint-planes,  and  that  the  presence  of  these  naturally 
influences  the  circulation.  When  all  the  porous  beds  in  a, 
series  of  strata  are  fully  saturated,  the  water  will  follow  the 
normal  direction.  But  when  continued  dry  conditions  have 
cut  off  the  supply  from  above,  and  the  discharge  by  springs 
begins  to  diminish,  water  seeks  its  way  down  through  joints 
and  fissures  from  one  porous  bed  to  another.  Hence  the 


FIG.  129.— DRAINAGE  IN  ANTICLINAL  STRATA. 

springs  issuing  from  the  deepest  beds  will  continue  to  yield 
their  usual  supply  long  after  the  highest  lying  springs  have 
failed.  The  exhaustion  of  the  springs  may  be  still  further 
delayed  by  the  exuding  of  water  from  the  less  porous  beds — 
all  of  which,  although  spoken  of  as  impermeable,  are  yet 
capable  of  absorbing  and  giving  out  water  in  less  or  greater 
degree. 

Springs  are  not  less  characteristic  of  massive  eruptive 
rocks  than  of  sedimentary  beds,  but  while  the  underground 
drainage  of  the  latter  is  conducted  principally  through  porous 
strata,  and  therefore  follows  a  determinate  direction,  that  of 
the  former  keeps  to  the  clefts  and  fissures,  and  as  these  vary 
in  width  and  trend,  and  may  be  numerous  in  some  places 
and  far  apart  elsewhere,  one  never  can  tell  where  springs 
are  likely  to  appear  at  the  surface.  Rain  falling  upon  a 
granite  mass  finds  its  way  down  through  innumerable 
fissures,  and  after  a  relatively  short  downward  course  may, 

z 


35,4  STRUCTURAL  AND  FIELD  GEOLOGY 

under  the  influence  of  gravity  alone,  escape  to  the  surface. 
Or  after  penetrating  to  a  great  depth — far  below  the  level, 
perhaps,  of  any  valley  in  the  neighbourhood — it  may  have 
its  passage  impeded  or  barred  by  the  closeness  of  the  joints. 
Subject  to  great  hydrostatic  pressure,  it  will  now  be  forced 
to  ascend  to  the  surface  along  the  same  kinds  of  fissures  by 
which  it  travelled  downwards,  and  will  issue  as  a  deep-seated 
spring,  which  may  or  may  not  have  a  temperature  exceeding 
that  of  the  region  where  it  appears.  It  is  not  improbable, 
indeed,  that  meteoric  water  may  sometimes  descend  by 
fissures  to  such  depths  that  its  further  progress  downward 
is  arrested  by  the  internal  heat  of  the  earth.  The  enormous 
pressure  at  a  depth  of  several  thousand  feet  will  prevent 
ebullition,  but  expansion  must  result,  and  this,  added  to  the 
hydrostatic  pressure  of  the  descending  currents,  will  force 


FIG.  130. — DRAINAGE  IN  MASSIVE  IGNEOUS  ROCK. 

the  water  through  other  fissures  to  the  surface.     Deep-seated 
springs  of  such  a  nature  might  either  be  cold  or  thermal 

(Fig.  130). 

The  underground  drainage  of  schistose  rocks  is  usually 
just  as  hard  to  determine  as  that  of  massive  eruptives. 
Exceptionally,  where  schists  are  relatively  well  bedded  and 
consist  of  a  series  of  rocks,  some  of  which  are  better  water- 
bearers  than  others,  springs  will  tend  to  appear  at  the  out- 
crops of  the  latter.  As  a  rule,  however,  owing  to  the 
abundant  folding  and  the  irregular  jointing,  the  direction 
of  the  underground  drainage  among  schistose  rocks  is  quite 
indeterminate. 

A  large  number  of  strong  springs  often  appear  along  the 
line  of  junction  between  an  intrusive  mass  and  the  rocks 
it  traverses.  The  water  may  be  derived  either  from  the  one 
or  the  other,  or  from  both.  In  Fig.  131,  for  example,  a  great 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE  355 

mass  of  granitoid  rock  is  represented  cutting  across  a  series 
of  relatively  impervious  strata.  The  rain  passes  downwards 
through  the  much-jointed  eruptive  mass,  where  it  accumulates. 
The  water  cannot  escape  laterally,  because  it  is  dammed  back 
by  the  impervious  beds  ($) ;  it  therefore  continues  to  accumu- 


FIG.  131. — HEAPING-UP  OF  WATER  IN  IGNEOUS  ROCK. 

late  until  it  reaches  the  point  where  the  line  of  junction 
comes  to  the  surface.  Here,  under  hydrostatic  pressure,  it 
flows  out  as  a  more  or  less  copious  spring  (s)  or  line  of 
springs.  In  many  cases,  however,  the  water  is  derived  from 
the  stratified  rocks  rather  than  the  intrusive  mass  by  which 
they  are  intersected.  In  Fig.  132,  the  strata,  consisting  of 


FIG.  132.— INTERCEPTION  OF  UNDERGROUND  DRAINAGE  BY 
INTRUSIVE  ROCK. 

pervious  and  impervious  rocks,  dip  towards  the  igneous  mass. 
Water  soaking  through  the  porous  beds  (p)  is  dammed  back 
by  the  basalt  (b)  and  forced  to  the  surface  (s)  along  the 
junction-line.  Now  and  again,  the  water  discharged  under 
such  conditions  would  seem  to  come  from  the  rocks  on  both 
sides  of  the  junction,  especially  when  the  igneous  mass  is 
of  the  nature  of  a  batholith,  and  exposed  at  the  surface  over 
a  considerable  area. 

Springs,  as  we  have  already  seen,  are  often  associated 


356 


STRUCTURAL  AND  FIELD  GEOLOGY 


with  those  vertical  wall-like  intrusions,  known  as  dykes  (see 
p.  202).  When  dykes  cut  across  inclined  rocks  in  the  general 
direction  of  the  strike,  they  naturally  act  as  subterranean 
dams,  interrupting  the  underground  water  flowing  towards 
them,  and  forcing  it  to  rise  to  the  surface  (Fig.  133). 


FIG.  133. — INTERCEPTION  OF  UNDERGROUND  DRAINAGE  BY  DYKE. 

Junction-springs  of  this  kind  are  very  common  in  Scotland. 
Probably  the  most  important  springs  of  all,  however,  are  those 
that  appear  on  lines  of  faulting  or  dislocation.  These,  as  we 
have  learned,  frequently  bring  permeable  against  impermeable 
rocks,  and  many  of  the  largest  dislocations  run  in  the  direction 
of  the  strike  of  inclined  strata.  Hence,  when  a  series  of 
porous  strata  dip  at  a  low  angle  for  a  long  distance,  towards 
one  of  these  faults,  on  the  other  side  of  which  the  rocks  are 
more  or  less  impermeable,  all  the  conditions  favour  the 
formation  of  strong  springs  (Fig.  1 34).  Even  when  the  strata 


FIG.  134. — INTERCEPTION  OF  UNDERGROUND  DRAINAGE  BY  FAULT. 

on  both  sides  of  a  fault  are  porous,  springs  will  usually 
indicate  its  presence,  for  faults  are  often  rilled  or  lined  with 
clay,  etc.,  and  thus  form  more  or  less  impervious  barriers, 
while  the  adjacent  rocks,  commonly  much  fissured  and  frac- 
tured, afford  the  underground  water  a  ready  passage  to  the 
surface.  A  fault  traversing  horizontal  strata  in  such  a  way 
as  to  bring  permeable  and  impermeable  rocks  into  juxta- 
position likewise  causes  springs  to  appear,  whenever  the 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE  357 

water-level  in  the  porous  beds  reaches  the  point  where  the  fault 
touches  the  surface  (Fig.  135) — the  conditions  being  somewhat 
comparable  to  those  represented  in  Fig.  131.  Considerable 
dislocations,  indeed,  will  usually  carry  water,  even  although 
the  structure  of  the  rocks  they  traverse  may  not  seem  very 
promising.  For  it  will  rarely  happen  that  a  normal  fault, 
cutting  across  a  great  thickness  of  rock,  will  not,  in  some 
parts  of  its  course,  truncate  water-bearing  beds,  the  fluid 
contents  of  which  are  under  sufficient  hydrostatic  pressure  to 
rise  to  the  surface.  The  faults  themselves  are  often  to  some 
extent  open  fissures  or  filled  with  rock-rubbish  which  is 
easily  penetrated ;  while  the  contiguous  rocks  on  one  or  both 
sides  are  usually  more  or  less  fractured  and  jumbled :  it  is 
not  surprising,  therefore,  that  springs  should  occur  along  lines 


FIG.  135.— HEAPING-UP  OF  WATER  IN  HORIZONTAL  STRATA. 

of  dislocation,  under  the  most  diverse  conditions.  It  will 
thus  be  seen  that  a  knowledge  of  the  faults  of  a  district  is 
highly  desirable,  if  we  would  understand  its  subterranean 
hydrography. 

Springs  are  usually  classified  as  shallow  or  surface  and 
deep-seated.  A  spring  which  fluctuates  with  the  seasons — tepid 
in  summer  and  cold  in  winter,  and  running  full  or  drying  up 
according  as  the  rainfall  is  excessive  or  scanty — is  obviously 
quite  superficial,  the  water  having  come  no  great  distance. 
Between  temporary  springs  of  this  type  and  perennial  springs 
whose  volume  remains  practically  constant,  and  whose  tem- 
perature does  not  vary  with  the  seasons,  there  are  all 
gradations.  Obviously,  the  most  persistent  springs  derive 
their  supplies  from  wide  gathering  grounds,  those  whose 
surface  never  rises  nor  falls  probably  coming  from  the  greater 
depths. 


358  STRUCTURAL  A^7n  FIELD  GEOLOGY 

Wells, — The  foregoing  rapid  sketch  of  the  conditions  that 
determine  the  underground  circulation  of  water  and  its  dis- 
charge at  the  surface  will  suffice  to  indicate  what  course  we 
should  follow  in  searching  for  a  subterranean  water-supply. 
From  the  very  earliest  times,  men  have  dug  for  water — a 
common  well  being  simply  a  hole  sunk  below  the  water-level, 
into  which  percolation  from  the  surrounding  rocks  takes  place. 
In  these  advanced  days,  we  now  imitate  Nature  on  a  bolder 
scale,  and  by  means  of  our  boreholes  produce  more  or  less 
deep-seated  perennial  springs.  Water  is  doubtless  very 
generally  distributed  through  the  superficial  parts  of  the 
earth's  crust,  but  all  rocks,  as  we  have  learned,  are  not 
equally  absorbent,  and  the  depth  of  the  water-level  from  the 
surface  is  very  variable.  It  is  obvious,  therefore,  that  before 
proceeding  to  sink  a  common  well,  we  should  first  ascertain 
whether  the  geological  conditions  are  favourable  or  not.  If 
the  rocks  of  the  district  be  highly  jointed  and  pervious,  we  are 
unlikely  to  succeed  ;  but  if  they  be  less  fissured,  there  is  some 
hope  of  reaching  the  water-level  at  a  moderate  depth.  It 
is  needless,  however,  to  say  that  much  will  depend  upon 
the  climatic  conditions  of  the  region,  for  the  position  of 
the  water-level  is  necessarily  largely  determined  by  the 
rainfall. 

The  superficial  accumulations  of  this  and  many  other 
countries  not  infrequently  contain  large  quantities  of  water, 
either  derived  directly  from  the  rainfall  or  introduced  into 
them  by  natural  springs — while  in  other  cases,  it  has  filtered 
into  them  from  streams.  Sheets  of  sand,  for  example,  which 
are  underlaid  and  perhaps  overlaid  by  impervious  clay, 
usually  hold  water.  Again,  the  recent  alluvia  of  our  rivers, 
and  the  more  ancient  flats  and  terraces  of  similar  materials 
which  occur  so  frequently  at  various  levels  in  our  valleys,  may 
yield  copious  supplies.  The  water  obtained  from  the  younger 
alluvia  has  obviously  percolated  into  them  from  the  adjacent 
stream  or  river.  The  older  alluvia,  on  the  other  hand,  have 
usually  derived  theirs  from  the  valley  slopes — a  very  superficial 
supply — but  now  and  again  the  water  flows  into  them  from 
true  springs  issuing  into  or  underneath  the  deposits.  Common 
wells,  dug  in  superficial  deposits  of  the  kind  referred  to,  not 
infrequently  yield  a  good  supply  of  potable  water,  but  they 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE   359 

are  not  always  to-be  trusted.  Near  towns  and  villages,  and 
even  in  the  vicinity  of  isolated  dwellings,  they  are  liable  to 
contamination,  impurities  being  readily  filtered  into  them, 
especially  during  wet  seasons.  The  wells  sunk  in  an  old  river- 
terrace  may,  under  ordinary  conditions,  yield  excellent  water, 
more  particularly  when  the  parent  source  of  the  supply  is  a 
spring  discharging  into  the  deposits.  In  times  of  heavy 
flood,  however,  when  the  adjacent  river  rises  to  the  level  of 
the  terrace,  the  gravel  is  rapidly  saturated,  and  impurities 
may  be  washed  into  the  wells.  The  fact  that  when  the  rivers 
are  in  flood,  outbreaks  of  typhoid  fever  often  occur  in  riparian 
districts  supplied  from  such  wells  is  sufficiently  suggestive. 
The  water,  which  under  ordinary  conditions  is  quite  whole- 
some and  suitable  for  all  domestic  purposes,  is,  perhaps,  never 
suspected,  and  may  continue  to  be  used  until  the  next  con- 
siderable flood  repeats  the  work  of  its  predecessor. 

In  certain  districts  deeply  covered  with  loose  deposits  of 
gravel,  sand,  clay,  etc.,  it  is  often  difficult  or  practically 
impossible  to  sink  common  wells  for  a  local  water-supply. 
When  such  is  the  case,  engineers  often  have  recourse  to 
driven  wells.  These  are  made  by  forcing  down  a  strongly 
pointed  iron  pipe,  pierced  with  holes  round  the  bottom  to 
admit  the  water.  The  advantages  of  this  system  are  obvious, 
for  not  only  can  the  pipe  be  driven  to  depths  much  below 
those  reached  by  any  ordinary  well,  but  the  water-supply 
obtained  is  protected  from  impurities  coming  from  the  surface. 
In  populous  districts,  however,  even  driven  wells  may  in  time 
become  polluted,  for  the  pipe,  subject  to  the  corrosive  action 
of  foul  liquids  descending  from  the  surface,  may  eventually 
yield  admission  to  the  enemy. 

When  good  springs  are  not  available,  common  wells  are 
often  the  only  source  of  supply  in  country  districts.  In 
sinking  these  it  is  always  advisable  to  take  the  geological 
conditions  into  consideration.  Remembering  that  under- 
ground water  finds  its  way  in  the  direction  of  dip,  care 
should  be  taken  to  sink  wells  in  such  positions  that  impure 
surface  water  cannot  reach  them  by  percolating  along  the 
bedding-planes.  It  is  absolutely  necessary,  moreover,  that 
wells  should  be  placed  as  far  away  as  possible  from  dwelling- 
houses,  cesspools,  drains,  etc.,  and  every  possible  source  of 


360 


STRUCTURAL  AND  FIELD  GEOLOGY 


pollution.  Frequently,  indeed,  it  may  be  found  necessary 
to  line  them  with  water-tight  walls,  especially  in  the  case 
of  wells  that  are  sunk  in  more  or  less  unconsolidated  deposits. 
Even  after  every  precaution  is  taken,  however,  no  surface- 
well  can  be  considered  perfectly  safe — although  there  are 
some  obviously  more  liable  to  be  contaminated  than  others. 

Wells  sunk  in  river  alluvia,  in  prox- 
imity to  and  upon  the  same  level  as 
dwelling-houses,  are  the  most  danger- 
ous of  all. 

Artesian  Wells. — The  driven  wells 
referred  to  above  are  often  true  arte- 
sian wells  on  a  small  scale,  for  an 
artesian  well  is  simply  a  borehole 
sunk  to  some  permeable  stratum  in 
which  the  water  is  under  such  high 
pressure  that  when  it  is  reached  it 
rises  towards  the  surface — the  upper 
limit  reached  by  it  being  determined 
by  the  height  of  its  head  or  source 
above  the  mouth  of  the  well,  and  the 
amount  of  frictional  resistance  it  has 
to  overcome.  Should  the  latter  be 
very  great,  there  may  be  no  rapid 
rise  of  water  in  the  well.  The  rock 
may  be  porous  enough  in  texture,  but 
if  it  be  not  traversed  by  more  or  less 
open  joints  and  fissures,  the  hydro- 
static pressure  may  barely  suffice  to 
keep  up  a  gentle  circulation.  Fortu- 
nately, such  planes  of  division  are 
seldom  or  never  absent,  and  in  cer- 
tain rocks,  as  we  have  learned,  they 
are  often  relatively  wide  and  open. 
The  accompanying  diagram  (Fig. 
136),  will  serve  to  indicate  the  geological  conditions  under 
which  an  artesian  water-supply  is  obtained.  The  section  is 
supposed  to  be  taken  across  a  broad  area,  throughout  which 
the  strata,  consisting  of  pervious  (£)  and  impervious  (imp) 
beds,  are  arranged  in  a  basin-shaped-  form.  Rain  falling  on 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE  361 

the  outcrops  of  the  pervious  beds  which  are  overlaid  and 
underlaid  by  impermeable  strata,  percolates  in  the  direction 
of  the  bedding-planes,  and  accumulates  until  each  porous 
stratum  becomes  saturated  up  to  its  outcrop.  It  is  obvious 
that  this  imprisoned  water  must  be  under  hydrostatic  pres- 
sure, which  necessarily  increases  with  the  depth  and  reaches 
a  maximum  in  the  centre  of  the  basin.  Were  a  boring  made 
at  I  through  the  uppermost  impervious  stratum  into  the 
subjacent  water-logged  bed,  an  uprush  of  water  to  the  sur- 
face would  ensue.  At  first,  the  water  might  form  a  tall 
fountain — the  height  of  which  would  be  determined  not  only 
by  hydrostatic  pressure,  but  by  the  amount  of  frictional 
resistance  to  be  overcome  by  the  water.  If  the  passage  of 
the  water  through  the  porous  bed  were  favoured  by  open 
fissures,  the  fountain  might  reach  a  height  not  very  much 
below  that  of  the  outcrop  of  the  bed.  Shortly,  however,  it 
would  begin  to  decrease  in  height  until  it  reached  a  level 
determined  by  the  average  rainfall  of  the  district.  It  would, 
in  fact,  behave  like  a  perennial  spring.  A  boring  sunk  at 
2  would  tap  a  deeper  stratum,  and  cause  a  still  stronger 
outflow  owing  to  the  greater  head  ;  while  the  beds  tapped 
by  3  and  4  would  for  a  similar  reason  send  yet  more 
powerful  uprushes  of  water  to  the  surface. 

The  geological  conditions  represented  in  the  diagram 
are,  of  course,  ideal.  Each  pervious  stratum  is  supposed  to 
retain  all  the  rain-water  which  soaks  into  it  at  its  outcrop. 
In  point  of  fact,  however,  such  conditions  rarely  if  ever  do 
obtain.  So-called  impervious  strata  are  only  relatively  non- 
porous,  "while  continuous  joints  and  other  lines  of  fissure, 
traversing  all  the  beds  of  a  series  alike,  are  so  seldom 
absent,  that  the  water  in  deeply  buried  pervious  beds  must 
in  less  or  greater  degree  escape  towards  the  surface.  Hence, 
when  an  artesian  well  is  sunk,  the  water  does  not  always 
rise  so  high  as  might  have  been  anticipated,  even  after 
allowance  has  been  made  for  frictional  resistance.  It  must 
not  be  supposed  that  a  basin-shaped  arrangement  of  the 
strata  is  essential  for  the  formation  of  artesian  wells.  Any 
series  of  impermeable  and  permeable  beds  dipping  continu- 
ously in  one  direction  for  some  considerable  distance,  may 
contain  abundant  supplies  which,  under  certain  conditions, 


362  STRUCTURAL  AND  FIELD  GEOLOGY 

can  be  reached   by  boring.     Some  of  these  conditions  may 
therefore  be  briefly  considered. 

A  well-jointed  porous  bed,  or  series  of  beds,  underlaid 
and  overlaid  by  impermeable  strata,  may, thin-out  gradually 
in  the  direction  of  dip,  and  when  such  is  the  case  they 
become  water-logged  (Fig.  137).  Or  the  descent  of  water 
along  the  bedding-planes  may  be  interrupted,  not  by  the 
thinning-out  of  beds,  but  by  such  barriers  as  have  already 
been  referred  to — faults,  dykes,  and  other  discordant  junctions. 
Or,  in  the  absence  of  any  underground  dams,  the  descending 
water  may  be  stopped  at  extreme  depths  by  the  increasing 
temperature  —  the  hydrostatic  pressure  being  eventually 
counterbalanced  by  the  tension  of  superheated  steam.  It 
is  under  such  conditions  as  these  that  many  natural  springs 


FIG.  137. — WATER-BEARING  BEDS  WEDGING  OUT  DOWNWARDS. 

originate — the  imprisoned  water  seeking  to  escape  pressure 
tends  to  rise  through  joints  or  other  lines  of  weakness  towards 
the  surface.  Sometimes,  however,  it  is  prevented  doing  so, 
or  is  only  partially  successful,  and  immense  stores  are  thus 
often  retained  underground.  Such  hidden  sources  are  not 
difficult  to  discover,  especially  in  regions  the  geological 
structure  of  which  can  be  readily  ascertained  from  a  study 
of  the  rocks  exposed  at  the  surface. 

The  more  important  points  to  be  considered  in  the 
search  for  an  artesian  water-supply  may  be  summarised  as 
follows : — 

i.  Having  ascertained  that  the  strata  over  a  wide  region 
have  a  dominant  dip  in  one  direction,  we  must  endeavour  to 
acquire  as  complete  a  knowledge  as  possible  of  the  various 
rocks  and  rock-groups.  It  is  essential  for  success  that 
pervious  beds  should  occur  interstratified  with  impermeable 
strata,  the  best  kinds  of  water-bearing  beds  being  sand  and 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE  363 

gravel,   sandstone,    grit,   conglomerate,   and    highly    fissured 
limestones. 

2.  The  thickness  of  the  entire  succession  of  strata  should 
be  carefully  measured,  and  the  precise  position  in  the  series 
of  any  water-bearing  beds  should  be  ascertained. 

3.  It  is,  further,  most  important  that  the  average  angle  of 
dip  should   be  determined,  otherwise  we  shall  be  unable  to 
estimate  the  depth  from   the   surface   at   which   the   water- 
bearing beds  may  be  expected  to  occur  at  any  given  point 

4.  The  inclination  of  the  strata  must  not  be  too  great,  for 
obvious   reasons : — (a)  Gently  inclined  strata  have  relatively 
broad  outcrops,  and  therefore  are   in   a  position  to  absorb 
more  rain  than  if  they  had  been  highly  inclined  or  vertical. 
Thus,  the  outcrops  of  a  series  of  strata,  100  feet  thick,  dipping 
at  an  angle  of  i°,  would  be  rather  more  than  a  mile  in  width ; 
while,  if  the  angle  of  dip  were  5°,  the  width  of  outcrop  would 
be   only    350  yards   or    thereabout — the    width    necessarily 
varying  inversely  as  the  dip ;  ($)  With  an  inclination  of  only 
i°,  a  stratum  descends  about  30  yards  in  a  mile ;  but  a  dip  of 
5°   carries    it   down   147   yards  in  the  same  distance  ;  while 
at  angles  of  10°,  20°,  and  40°,  the  depths  at  which  the  stratum 
would    occur   would    be    about    300,   630,   and     1490    yards 
respectively.     It  is  obvious,  therefore,  that  with   a   high  dip 
a  water-bearing  bed  must  within  a  short  distance  descend  to 
a  greater  depth  than  the  engineer  might  consider  it  possible 
or  desirable  to  bore. 

5.  Let  us  suppose,  however,  that  we  have  assured  ourselves 
that  the  character  of  the  strata  is  quite  satisfactory,  and  that 
their   inclination   is    equally   favourable ;    we    have    still    to 
ascertain  whether  the  region  is  traversed   by   faults,  dykes, 
or   other   discordant    junctions    which    may   serve    as    sub- 
terranean   dams.     It    is    quite    obvious    that,   if    any   such 
obstructions  occur,  their  position  must  influence  the  engineer 
in  selecting  a  site  for  an  artesian  well.     It  would  be  hopeless 
to  bore  for  water  on  the  dip-side  of  a  strike-fault  or  a  dyke 
following  approximately  the  same  direction,  whereas  a  boring 
put  down  on  the  rise-side  would  in  most  cases  be  successful. 

6.  When  the  strata  consist  throughout  of  pervious  beds — 
such  as  sandstones,  highly  cleft  limestones,  etc. — the  chances 
of  obtaining  an   artesian  water-supply  are  much  diminished, 


3f>4  STRUCTURAL  AND  FIELD  GEOLOGY 

Nevertheless,  even  in  such  cases,  there  will  in  all  probability 
be  some  closer  grained  or  relatively  impervious  bed  or  beds 
to  stay  the  downward  passage  of  the  water.  An  extended 
survey  of  adjoining  districts  may  even  show  that  the  pervious 
beds,  as  they  range  beyond  our  region,  are  underlaid  by 
or  interosculate  with  impervious  beds  ;  or  that,  in  the  direction 
of  the  dip,  they  are  eventually  interrupted  by  faults,  dykes, 
or  other  barriers,  and  may  not,  therefore,  be  so  barren  of 
water  at  a  moderate  depth  as  appearances  in  our  own 
neighbourhood  might  have  led  us  to  infer.  Should  the  whole 
area  be  more  or  less  deeply  mantled  with  impervious  super- 
ficial accumulations,  such  as  boulder-clay,  the  prospects  of 
obtaining  water  by  boring  would  be  considerably  increased. 

7.  It  goes  without  saying  that  the  water  obtained  from  a 
bore-hole,  although  sufficiently  abundant,  may  yet  be  unsuit- 
able for  the  purpose  the  engineer  has  in  view.  A  careful 
survey  of  the  catchment  area  should,  therefore,  be  made 
preliminary  to  any  boring.  It  may  be  that  some  of  the 
pervious  beds  contain  deleterious  ingredients,  which  must 
unfavourably  affect  the  quality  of  the  water ;  while,  at  other 
horizons,  water-bearing  strata  of  a  satisfactory  character 
occur.  Should  it  be  necessary  to  pass  through  an  undesirable 
source  of  supply  to  reach  a  more  promising  source  at  a  lower 
level,  the  water  coming  from  the  higher  level  can  be  prevented 
from  contaminating  that  of  the  lower  level,  by  simply  tubing 
it  off.  Another  danger  is  the  infiltration  of  foul  liquids  from 
the  surface.  These  may  not  penetrate  sufficiently  far  to 
affect  the  water  at  the  relatively  deep  level  from  which  it  is 
drawn.  But  unless  the  bore-hole  is  properly  tubed  for  some 
distance  down  from  the  surface,  it  is  likely  enough  to  be 
invaded  by  pollutions. 

Drainage. — In  planning  a  scheme  of  drainage  either  for 
town  or  country,  it  is  not  enough  to  consider  only  the  natural 
slope  of  the  ground,  and  the  ease  with  which  the  waste 
products  of  a  community  can  be  discharged  elsewhere.  It 
must  be  remembered  that  the  external  configuration  of  the 
ground  may  not  coincide  even  approximately  with  the 
internal  or  geological  structure.  The  surface  may  slope  in 
one  direction  and  the  subjacent  strata  in  quite  another. 
There  is  always  a  danger,  therefore,  of  polluted  liquid  finding 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE   365 

its  way  down  the  bedding-planes,  and  poisoning  the  under- 
ground water-supply  of  adjacent  districts.  It  is,  of  course, 
often  necessary  to  carry  drains  in  some  given  direction  other 
than  that  which  geological  considerations  might  dictate — for 
questions  of  safe  outlet  and  the  expense  of  excavation  cannot 
be  ignored.  In  cases  of  this  kind,  all  the  engineer  can  do  is 
to  see  that  the  drains  are  made  as  water-tight  as  may  be, 
and  to  insist  on  the  closing  of  every  common  well  in  the 
immediate  neighbourhood.  Flat  lands  in  the  neighbourhood 
of  considerable  towns  are  sometimes  laid  out  as  sewage-fields, 
with  satisfactory  results,  it  may  be,  to  the  towns,  but  often 
much  to  the  prejudice  of  a  scattered  rural  population,  whose 
water-supply  may  come  entirely  from  common  wells.  What- 
ever excuse  there  may  be  for  carrying  the  drainage  of  a 
city  in  some  particular  direction,  irrespective  of  geological 
considerations,  there  can  be  none  for  discharging  sewerage 
over  the  surface  of  the  ground  without  preliminary  inquiry 
as  to  what  may  happen  in  the  event  of  leakage  into  the  rocks 
below.  Cases  could  be  cited  to  show  how  neglect  of  this 
precaution  has  been  followed  by  disastrous  results.  Some 
low-lying  fields  were  selected  by  a  certain  town  for  sewage 
irrigation.  The  subsoil  was  boulder-clay,  a  deposit  supposed 
by  the  engineers  to  be  impervious.  But  there  are  boulder- 
clays  and  boulder-clays ;  many  are  practically  impermeable, 
others  are  only  relatively  so.  This  particular  boulder- 
clay  was  one  of  the  latter  class,  somewhat  sandy  in  texture, 
and  only  two  or  three  feet  at  most  in  thickness.  Unfor- 
tunately, also,  the  sewage-field  spread  over  the  back  of  a  low 
anticlinal  arch.  Under  those  conditions  the  inevitable  result 
followed  in  due  time.  In  a  year  or  so  the  subsoil  of  boulder- 
clay  become  water-logged,  leakage  took  place  into  the 
underlying  strata,  and  the  foul  liquid  made  its  way  down  the 
bedding-planes  and  poisoned  all  the  springs  and  common 
wells  in  the  surrounding  neighbourhood — typhoid  fever,  of 
course,  ensuing. 

In  villages  and  rural  districts,  where  no  general  system  of 
drainage  is  provided,  cesspools  are  often  sunk.  When  these 
are  carried  down  through  a  thick  bed  of  clay  into  underlying 
gravel  and  sand,  they  may  suit  the  purpose  of  their  owners 
well  enough.  It  is  obvious,  however,  that  they  are  a  menace 


366  STRUCTURAL  AND  FIELD  GEOLOGY 

to  the  neighbourhood,  and  that,  in  time,  springs  and  common 
wells  may  become  polluted.  This  primitive  kind  of  drainage 
is  only  permissible  in  new  countries  where  the  population  is 
sparsely  scattered.  They  should  not  be  tolerated  in  a 
populous  region  that  is  dependent  for  its  water-supply  on 
wells  and  springs,  without  the  most  careful  inquiry  into  the 
geological  conditions. 

Low-lying  lands  with  a  very  gentle  slope  are  often  cold 
and  wet,  or  even  swampy  and  boggy.  Surface  drains  in  such 
cases  may  be  quite  useless,  owing  to  the  low  gradients. 
Very  often  it  is  the  presence  of  an  impermeable  hard  pan 
at  the  depth  of  a  foot  or  two  below  the  surface,  which 
prevents  the  escape  of  the  water.  With  the  breaking-up  of 
this  "  pan,"  the  superficial  water  filters  into  underlying  porous 
beds,  and  the  land  is  at  once  improved.  Occasionally,  the 
barrier  to  the  escape  of  superficial  drainage  is  more  deeply 
seated,  and  may  be  due  to  the  occurrence  of  some  thick 
impervious  stratum  of  clay,vshale,  etc.,  either  occupying  the 
surface  or  appearing  immediately  underneath  overlying  porous 
beds.  In  such  a  case,  if  no  outlet  can  be  obtained  by  surface- 
draining,  it  is  sometimes  possible  to  sink  pits  or  to  bore 
through  the  impermeable  stratum  into  underlying  beds 
which  are  porous,  and  through  which  the  superficial  water 
eventually  drains  away. 

Distribution  of  Disease  in  Relation  to  Geological 
Conditions. — Some  writers  have  adduced  statistics  to  show 
that  certain  diseases  are  more  or  less  closely  associated  with 
particular  geological  systems.  This  supposed  connection 
between  groups  of  rocks  and  the  diseases  that  afflict  the  folk 
who  reside  upon  them,  is  due  rather  to  the  physical  conditions 
of  the  regions  where  those  rocks  are  prevalent  than  to  the 
rocks  themselves.  The  connection,  if  there  be  any,  is  not 
direct,  but  indirect.  In  most  cases  it  will  be  found  that  the 
more  immediate  causes  of  such  distribution  of  disease  are 
(a)  the  water  supply ',  and  (b)  the  physical  conditions,  more 
especially  those  which  affect  the  force  and  direction  of  the 
winds.  Many  diseases,  as  everyone  knows,  arise  from  the 
free  use  of  contaminated  water.  Some  water,  again,  may 
not  be  polluted  by  sewage,  and  yet  contain  an  excess  of 
inorganic  matter  (as  lime,  magnesia,  iron-oxide,  etc.)  in 


ECONOMIC  ASPECTS  OF  GEOLOGICAL  STRUCTURE  367 

solution,  or,  on  the  other  hand,  there  may  be  an  insufficient 
quantity  of  mineral  ingredients  present,  as  in  the  case  of 
water  derived  from  melting  snow  or  ice.  Other  districts  may 
be  unhealthy  owing  to  the  undrained  character  of  the  land. 
It  is  matter  of  common  knowledge  that  damp,  cold  soils 
favour  many  diseases.  Even  in  one  and  the  same  district, 
we  may  note  that  houses  built  on  dry  rock  are  healthier 
residences  than  others  in  their  proximity  which  are  founded 
on  damp  ground.  Once  more,  the  topographical  conditions 
of  a  district  necessarily  influence  the  climate  and  affect  the 
health  of  a  community.  It  has  been  found,  for  example,  that 
a  high  rate  of  mortality  prevails  in  districts  which  are  exposed 
to  the  full  force  of  the  winds  sweeping  inland  from  the  sea, 
the  mortality  being  largely  due  to  phthisis. 

If,  therefore,  it  be  ascertained  that  the  occupants  of  some 
particular  geological  area  are  more  subject  to  certain  diseases 
than  the  denizens  of  another  where  the  prevailing  rocks  are 
of  a  different  kind,  we  need  not  credit  the  rocks  themselves 
with  having  any  direct  influence  on  the  life  of  man.  In 
most  cases  it  will  be  found  that  districts  are  unhealthy  either 
because  of  insufficient  drainage,  or  the  water-supply  will  be 
to  blame,  or  it  may  be  that  the  topographical  conditions  do 
not  afford  sufficient  protection  from  the  full  force  of  the 
wind. 


CHAPTER  XXIV 

SOILS   AND   SUBSOILS 

Agents  of  Disintegration  ;  Insolation  and  Deflation  ;  Rain;  Frost;  Life. 
Weathering  of  Rocks.  The  Soil-cap.  Classification  of  Soils— I. 
Bed-rock  Soils,  their  Varied  Character  ;  Soils  derived  from  Igneous, 
Metamorphic,  and  Derivative  Rocks.  II.  Drift  Soils;  Glacial, 
Alluvial,  and  /Eolian  Soils. 

Rock-disintegration.— Subsoil  has  already  been  defined  as 
an  unconsolidated  heterogeneous  aggregate  of  disintegrated 
rock  material,  and  soil  as  essentially  the  same,  with  the 
addition  of  organic  matter.  Both  are  the  result  of  the 
operation  of  various  epigene  agents,  and  are  therefore 
properly  included  among  derivative  rocks.  Everywhere 
throughout  the  world  we  meet  with  these  superficial 
accumulations.  As  might  have  been  expected,  they  vary 
in  character  according  to  the  conditions  under  which  they 
have  been  formed,  and  the  nature  of  the  rocks  from  which 
they  have  been  derived.  In  some  regions,  for  example, 
they  consist  largely  of  angular  and  subangular  detritus,  while 
in  other  places  they  may  consist  mainly  of  sand  or  of  clay, 
as  the  case  may  be.  In  high  latitudes  and  in  mountainous 
lands  the  soil-cap  is  usually  very  stony;  in  lower  latitudes 
and  over  plains  and  gently  undulating  tracts  its  ingredients 
are  commonly  finer  grained.  In  most  regions,  however, 
arable  soils  are  composed  essentially  of  insoluble  quartz-sand 
and  clay,  in  ever-varying  proportions,  throughout  which  are 
disseminated  larger  or  smaller  percentages  of  organic  matter 
and  of  certain  more  or  less  soluble  ingredients,  such  as  iron- 
oxides,  magnesia,  lime,  soda,  potash,  etc. 

In  preceding  pages,  frequent  reference  has  been  made  to  the 
alterations  and  transformations  of  minerals  and  rocks  induced 


SOILS  AND  SUBSOILS  369 

by  epigene  action.  Again,  while  deaKng  with  the  structural 
phenomena  of  derivative  rocks,  we  have  briefly  considered 
the  various  origin  of  sedimentary  deposits.  The  remarks 
that  follow,  therefore,  may  be  taken  as  a  kind  of  summary 
of  much  that  has  already  been  advanced  on  the  subject  of 
rock-disintegration,  illustrated  by  special  reference  to  the 
geological  origin  of  soils  and  subsoils. 

Insolation  and  Deflation. — Among  the  various  agencies 
that  tend  to  disintegrate  rocks,  and  to  form  a  soil-cap,  are 
changes  of  temperature.  Rock-surfaces  are  heated  by  day 
and  in  summer  time — cooled  at  night  and  during  winter. 
They  thus  alternately  expand  and  contract,  and  this  leads 
to  disintegration,  for  their  constituent  minerals  often  yield 
unequally  to  strain  or  tension.  Such  is  notably  the  case 
with  rocks  composed  of  minerals  differing  in  colour,  density, 
and  expansibility,  such  as  granite,  gneiss,  diorite,  etc.  Even 
in  the  case  of  homogeneous  rocks,  it  is  obvious  that  alternate 
heating  and  cooling  of  the  surface  must  give  rise  to  strain 
and  tension.  In  countries  like  our  own,  where  there  is  no 
great  diurnal  range  of  temperature,  any  rock-changes  due 
to  this  cause  alone  are  hardly  noticeable,  since  they  are 
masked  or  obscured  by  the  action  of  other  and  more  potent 
agents.  But  in  the  rocky  deserts  of  tropical  and  subtropical 
regions,  bare  of  verdure  and  practically  rainless,  the  effects 
produced  by  alternate  heating  and  cooling,  or  "  insolation " 
as  the  process  is  termed,  are  very  marked.  The  rocks  are 
cracked  and  shattered  to  a  depth  of  several  inches ;  the 
surfaces  peel  off  and  are  rapidly  disintegrated  and  pulverised. 
Wind  then  catches  up  the  loose  material  and  sweeps  it  away, 
leaving  fresh  surfaces  exposed  to  the  same  destructive 
action.  More  than  this,  the  grit,  sand,  and  dust  carried  off 
by  the  wind  are  used  as  a  sand-blast  to  abrade  and  erode 
the  rocks  against  which  they  strike.  In  this  manner  cliffs 
and  projecting  rocks  are  undermined,  and  masses  from  time 
to  time  give  way  and  fall  to  the  ground,  where,  subject  to 
the  same  grinding  action,  especially  towards  the  base,  they 
tend  to  assume  the  appearance  of  irregular  blocks  supported 
upon  pedestals.  "  Deflation,"  or  the  transporting  action  of 
the  wind,  goes  on  without  ceasing,  with  the  general  result 
that  the  whole  surface  of  a  rainless  region  tends  to  be 

2  A 


370  STRUCTURAL  AND  FIELD  GEOLOGY 

gradually    lowered,  the   loose   materials   travelling   outwards 
from  the  scene  of  their  origin  to  the  borders  of  the  desert. 

Action  of  Rain. — Even  in  countries  like  our  own,  insola- 
tion   and   deflation  doubtless  share  in  the  disintegration   of 
rocks    and   the   transport   of   the   loosened    materials.     Un- 
doubtedly, however,  in  these  latitudes,  the  most  conspicuous 
agents   employed    in   the   work   of    reducing    rocks    to    the 
condition  of  grit,  sand,  and  clay,  are   rain  arid   frost.     Rain 
always  contains  some   oxygen    and   carbonic   acid    absorbed 
from  the  atmosphere,  and  after  it  reaches   the   ground,  still 
larger  stores  are  derived  by  it  from  the  decaying  vegetable 
and  animal  matter  with  which  soils  are  more  or  less  impreg- 
nated.    It  is  thus  enabled  to  attack  the   minerals   of  which 
rocks  are  composed.     In  every  region,  therefore,  where  rain 
falls,  soluble  rocks,  such  as  limestone,  are  gradually  dissolved, 
while  other  kinds  of  rock  are  decomposed  and  disintegrated. 
In  limestone  areas  it  can  be  shown  that  sometimes  hundreds 
of  feet  of  rock  have  thus  been  gradually  removed  from  the 
surface   of  the    land.     And  the  great  depth  now  and  again 
attained    by  rotted    rock   testifies   likewise   to   the   chemical 
activity   of   rain-water.      This    is   particularly   noticeable    in 
warm-temperate,  sub-tropical,  and   tropical    latitudes,   where 
felspathic  rocks  are  not  infrequently  decomposed  to  depths  of 
a  hundred  feet  or  more.     In  temperate  and  northern  regions, 
the  amount  of  rotted  rock  is  rarely  so  great.     The   thicker 
rock-crusts  of  southern  latitudes  are  supposed  to  be  due  to 
the  larger  supplies  of  acid  derived  from  the  more  abundant 
vegetation.     To  some  extent  this  is  probably  true,  and  there 
can  be  little  doubt,  also,  that  the  chemical  action  of  rain  is 
facilitated  by  the  higher  temperature  of  those  regions.     There 
is  another  reason,  however,  for  the  relatively  meagre  develop- 
ment of  rotted  rock  in  northern  countries  generally.     Those 
regions  have,  at  a  geologically  recent  date,  been  subjected  to 
glacial   conditions.     Broad  areas  of  temperate    Europe    and 
North    America,  for    example,  have   been    scraped   bare   by 
extensive  ice-sheets,  resembling  those  of  Greenland  and  the 
Antarctic  Circle.     In  more  southern  latitudes  the  rotted  rocks 
have  escaped  such  abrasion  and  denudation,  and  hence  it  is 
not   strange   that   they  should    attain    so   great   a  thickness. 
The  decomposed  rock-material  encountered  in  the  northern 


SOILS  AND  SUBSOILS  371 

parts  of  Europe  and  North  America  has  been  formed,  for  the 
most  part,  subsequent  to  the  disappearance  of  glacial  con- 
ditions ;  while  in  southern  regions,  rock  decay  has  gone  on 
without  interruption  ever  since  those  lands  came  into 
existence. 

Action  of  Frost. — The  disintegrating  action  of  rain  in 
temperate  and  high*  latitudes  is  greatly  aided  by  frost,  and 
the  same  is  the  case  in  the  elevated  tracts  of  southern 
latitudes.  Rain  renders  the  superficial  parts  of  rocks  more 
porous,  and  thus  enables  frost  to  act  more  effectually ;  while 
frost,  by  widening  pores  and  fissures,  affords  readier  ingress 
to  meteoric  water.  Water  freezing  in  soils  and  subsoils,  and 
in  the  interstitial  pores  and  minute  fissures  of  rocks,  forces 
the  grains  and  particles  asunder,  and  when  thaw  ensues,  the 
loosened  material  is  ready  to  be  carried  away  by  rain  or 
melting  snow,  and  subsequently,  it  may  be,  by  wind.  The 
same  process  takes  place  on  a  larger  scale  in  the  prising-open 
of  joints  and  bedding-planes,  and  the  consequent  rending 
asunder  of  rocks.  This  action  is  best  seen  in  Arctic  regions 
and  at  high  levels  in  our  own  and  other  countries,  where  the 
solid  rocks  not  infrequently  become  buried  underneath  their 
own  ruins.  By  and  by,  however,  these  loose,  angular  frag- 
ments are  shattered,  crumbled,  and  pulverised  by  frost,  until 
they  are  in  a  condition  to  be  swept  away  by  wind,  rain,  or 
melting  snow.  The  solid  rock  then  comes  again  within  reach 
of  the  same  destructive  action,  and  so  the  work  of  disruption 
and  disintegration  continues. 

Action  of  Plants  and  Animals. — The  acids  derived  from 
decaying  organic  matter  are  powerful  agents  of  chemical 
change.  Without  their  aid,  rain  would  be  a  much  less  effec- 
tive worker.  Living  plants  themselves,  however,  attack 
rocks,  and  by  means  of  the  acids  in  their  roots,  dissolve  out 
the  mineral  matter  required  by  the  organisms.  Further, 
their  roots  penetrate  the  natural  division-planes  of  rocks  and 
wedge  them  asunder ;  and  thus,  by  allowing  freer  percolation 
of  water  they  prepare  the  way  for  more  rapid  disintegration. 
Nor  can  we  overlook  the  part  played  by  tunnelling  and 
burrowing  animals,  some  of  which  aid  considerably  in  the 
work  of  reducing  rocks.  Worms,  for  example,  by  triturating 
in  their  gizzards  the  stony  particles  of  a  soil,  reduce  these  in 


372  STRUCTURAL  AND  FIELD  GEOLOGY 

size.  They  also  play  a  most  important  part  in  soil-circulation. 
In  soils  which  have  long  been  undisturbed  by  the  plough, 
coarse  particles  and  stones  are  usually  absent.  This  is 
obviously  due  in  chief  measure  to  the  bringing  up  by  the  worms 
of  fine  soil  from  below,  and  its  deposition  at  the  surface  as 
"  casts,"  which  are  spread  out  by  the  action  of  rain  and  wind. 
Eventually,  in  this  way,  a  more  or  less  considerable  stratum  of 
fine  soil  accumulates,  and  gradually  buries  any  stones  that 
may  have  been  lying  at  the  surface.  It  must  be  remembered, 
however,  that  the  transport  of  dust  by  wind  is  also  an 
important  factor  in  the  formation  of  fine  soil  in  many  regions, 
and  that  the  gradual  burial  of  "  ancient  monuments  "  of  one 
kind  or  another  is  probably,  in  many  cases,  largely  due  to 
the  gradual  accretion  of  wind-blown  materials. 

Weathering  of  Rocks. — We  have  now  enumerated  the 
more  important  epigene  agents  employed  in  the  formation  of 
soils  and  subsoils.  As  these  several  agents  are  often 
associated  in  their  work,  it  is  sometimes  hard,  or  even 
impossible,  to  say  which  has  played  the  dominant  role  in 
certain  cases.  It  is  obvious,  however,  that  the  disintegration 
of  rocks  is  partly  a  mechanical,  partly  a  chemical,  process, 
and  that  the  ultimate  result  of  superficial  action  is  to  break 
up  minerals  and  rocks  into  soluble  and  insoluble,  or  practically 
insoluble  ingredients.  Even  the  hardest  and  most  resistant 
of  rocks  and  rock-ingredients  must  succumb.  Those  that 
resist  solution  are  eventually  reduced  by  mechanical  action 
to  finely  divided  particles,  which  are  readily  transported  by 
running  water  or  carried  on  the  wings  of  the  wind.  Some  of 
the  harder  minerals,  and  notably  quartz,  may  long  survive  the 
rock  masses  of  which  they  once  formed  a  constituent  portion, 
and  continue  to  play  the  same  part  over  and  over  again. 
Here,  for  example,  is  a  pebble  of  liver-coloured  quartz  picked 
up  from  a  gravelly  beach  on  the  Firth  of  Forth.  Whence  has 
it  come,  and  what  tale  has  it  to  tell  ?  Originally  it  formed  a 
portion  of  some  vein  or  layer  traversing  the  metamorphosed 
rocks  of  the  Scottish  Highlands.  Detached  from  its  parent 
rock  in  Old  Red  Sandstone  times,  it  was  rolled  down  the  bed 
of  some  torrential  stream,  becoming  well  rounded  in  the 
process,  until  it  reached  the  shore  of  a  great  inland  sea — 
the  "  Lake  Caledonia "  of  geologists.  Together  with  many 


SOILS  AND  SUBSOILS  373 

boulders  and  pebbles  of  the  same  kind,  and  multitudinous 
rounded  stones  of  other  types  of  rock,  it  eventually  became 
sealed  up  in  the  great  conglomerate  that  forms  the  base 
of  the  Old  Red  Sandstone  system  in  Central  Scotland. 
Ages  pass  away  —  Lake  Caledonia  vanishes,  and  its 
conglomerates,  red  sandstones,  etc.,  and  igneous  rocks,  now 
forming  part  of  a  land-surface,  are  gradually  denuded.  The 
old  conglomerate  is  largely  broken  up,  and  our  liver-coloured 
quartz,  again  at  liberty,  becomes  the  sport  of  the  waves  upon 
a  sea-beach  of  Carboniferous  times.  Reduced  in  size  by 
constant  attrition,  but  otherwise  unchanged,  it  is  eventually 
locked  up  in  one  of  the  numerous  conglomerates  of  the 
period.  What  its  history  may  have  been  throughout  the 
vast  aeons  which  succeeded  up  to  the  close  of  Tertiary 
times,  we  cannot  tell.  Possibly  it  lay  perdu  during  all  that 
prolonged  period  in  its  Carboniferous  bed.  Or  it  may  have 
been  dug  out  at  some  distant  date,  and  again  played  its  part 
as  a  rolling-stone  on  sea-beach  or  river-floor.  Eventually, 
however,  it  was  enclosed  in  the  bottom-moraine  or  boulder- 
clay  of  the  great  mer  de  glace  that  formerly  overwhelmed  all 
Scotland.  In  due  time  this  mer  de  glace  vanished,  leaving 
its  accumulations  to  be  attacked  and  disintegrated  in  their 
turn.  Nowadays,  the  boulder-clay  is  being  eaten  into  by  the 
sea,  and  our  liver-quartz,  once  more  set  free,  repeats  its  coastal 
wanderings,  and  for  all  that  we  can  tell  may  yet  survive  to 
run  through  a  similar  cycle  of  changes  again  and  yet  again. 

But  quartz  is  an  exceptional  mineral,  in  comparison  with 
which  the  great  majority  of  rock-formers  are  ephemeral. 
Few  of  the  numerous  complex  silicates  with  which  it  is 
associated  in  crystalline  igneous  and  metamorphic  rocks 
survive  the  process  of  disaggregation  by  which  these  gradu- 
ally become  broken  up.  Now  and  again  the  felspars,  and 
even  some  of  their  ferromagnesian  associates — all  in  a  more 
or  less  altered  state — may  yet  retain  their  individuality,  and 
enter  sparingly  or  more  abundantly,  as  the  case  may  be,  into 
the  composition  of  derivative  rocks.  In  the  case  of  arkose, 
for  example,  we  have  a  rock  derived  immediately  from  the 
disaggregation  of  a  granite,  and  consisting,  therefore,  of 
quartz,  felspar,  and  mica,  assorted  and  arranged  by  water 
action.  The  quartz  may  be  more  of  less  water-worn,  and  the 


374  STRUCTURAL  AND  FIELD  GEOLOGY 

felspar  and  mica  not  only  worn,  but  to  some  extent  chemically 
altered — nevertheless,  each  mineral  has  retained  its  individu- 
ality. In  like  manner,  certain  of  the  minor  accessory 
ingredients  of  crystalline  igneous  rocks  have  often  survived 
the  demolition  of  their  parent  rocks,  and  are  now  and  again 
met  with  as  rolled  pebbles  and  grains  in  sand  and  gravel.  In 
such  cases,  however,  the  gravel  and  sand  have  usually  been 
derived  directly  from  the  disintegration  of  the  igneous  rocks 
in  question.  Neither  zircon,  rutile,  nor  magnetite  could 
survive  the  manifold  vicissitudes  through  which  grains  and 
pebbles  of  quartz  have  passed.  Sooner  or  later  they  lose 
their  individuality,  and  are  transformed. 

Thus  the  process  of  rock-disintegration  or  "weathering," 
as  it  is  termed,  may  be  said  to  consist  essentially  in  the 
breaking  up  of  complex,  and  therefore  usually  unstable 
compounds,  and  the  consequent  production  of  simpler  and 
more  stable  bodies.  Hence,  when  igneous  and  schistose 
rocks  are  highly  weathered,  their  complex  silicates  are  trans- 
formed and  converted  into  simpler  compounds,  some  of  which 
are  soluble,  while  others  are  more  or  less  insoluble.  The 
soluble  ingredients  tend,  therefore,  to  be  leached  out  and 
washed  away.  The  soil-cap  formed  upon  such  rocks,  how- 
ever, is  rarely  quite  destitute  of  soluble  matter.  Even  when 
the  disintegrated  materials  have  been  transported  by  water 
and  deposited  elsewhere  in  the  form  of  sand,  clay,  silt,  etc., 
these  sediments  will  usually  retain  a  larger  or  smaller 
proportion  of  soluble  matter — the  process  of  disintegration 
and  decomposition  of  the  several  constituents  of  the  original 
or  mother  rock  has  not  been  completed.  In  short,  sedi- 
mentary deposits,  derived  directly  from  the  breaking-up  of 
igneous  masses,  frequently  contain  a  larger  or  smaller 
proportion  of  the  relatively  unaltered  detritus  of  the  parent 
rocks.  We  know,  however,  that  many  sedimentary  rocks  are 
built  up  of  materials  which  have  been  used  over  and  over 
again.  Rocks  of  this  kind,  therefore,  may  consist  exclusively 
of  insoluble  ingredients — the  only  soluble  matter  they  may 
contain  being  the  binding  or  cementing  material  introduced 
into  them  by  percolating  water.  Repeatedly  exposed  to 
weathering — winnowed  and  rewinnowed  again  and  again  by 
wind  or  water,  or  both — sedimentary  materials  eventually 


SOILS  AND  SUBSOILS  375 

come  to  form  beds  of  pure  quartz-sand  and  fine  clay, 
composed  of  hydrous  silicate  of  alumina  alone,  with,  in 
most  cases,  some  proportion  of  the  finest  quartz-flour — all 
soluble  ingredients  having  been  removed. 

The  Soil-cap. — If  the  soil-cap,  therefore,  consists  essentially 
of  disintegrated  rock-materials,  it  is  obvious  that  it  must 
vary  very  much  in  character.  In  some  places  it  will  contain 
a  high  percentage  of  soluble  matter — in  other  places  it  will 
contain  little  or  none  at  all — and  between  these  extremes 
all  gradations  may  be  expected  to  occur.  The  character 
of  the  soil-cap  being  thus  dependent  upon  that  of  the 
underlying  rocks,  a  good  geological  map  might  be  expected 
to  throw  much  light  on  the  present  distribution  of  soils. 
And  so,  indeed,  it  does ;  nevertheless,  other  factors  have  had 
their  influence  upon  the  distribution  of  soils,  and  unless  these 
are  kept  in  view,  a  geological  map  may  be  misleading.  The 
colours  upon  such  maps  have  reference,  as  a  rule,  only 
to  the  so-called  "solid  rocks,"  and  these  may  or  may  not 
crop  out  at  the  actual  surface.  As  a  matter  of  fact,  they  are 
often  deeply  buried  underneath  superficial  accumulations  of 
gravel,  sand,  clay,  loam,  peat,  etc.  Wide  regions  may  be 
represented  on  the  map,  therefore,  as  being  occupied  by 
limestone,  or  by  sandstones  and  shales,  or  other  strata — 
although  none  of  these  may  actually  appear  at  the  surface — 
the  only  exposures  being  those  seen  in  stream-  and  river- 
courses.  All  the  intervening  low  grounds  may  be  thickly 
mantled  with  superficial  deposits.  In  cases  of  this  kind, 
therefore,  the  soils  take  their  character  from  the  overlying 
deposits,  and  not  from  the  covered  and  concealed  bed-rock. 
It  is  hardly  necessary  to  say  that  under  such  conditions  the 
soil-cap  may  differ  very  considerably  in  character  from  that 
which  the  solid  rocks  would  have  yielded.  Again,  the  actual 
configuration  of  the  ground  must  influence  the  distribution 
of  soil.  All  loose  disintegrated  rock  material  tends  to  travel 
downwards.  Rain,  alternate  frost  and  thaw,  etc.,  slowly  or 
more  rapidly,  as  the  case  may  be,  cause  soil-ingredients  to 
pass  from  higher  to  lower  levels — thus  the  disintegrated 
materials  derived  from  one  kind  of  rock  may  invade  and 
overlie  the  outcrops  of,  it  may  be,  totally  different  strata. 
The  character  of  a  soil  may  even  be  very  considerably 


376  STRUCTURAL  AND  FIELD  GEOLOGY 

modified  by  the  action  of  the  wind.  In  Central  France, 
for  example,  wind  blowing  from  east  or  south-east  is  laden 
with  fine  dust,  derived  from  the  disintegration  of  the  volcanic 
rocks  of  Mont  Dore  and  Cantal.  This  dust,  therefore, 
contains  many  fertilising  ingredients — notably  potash  and 
phosphoric  acid.  Brought  down  by  rain  and  snow,  it  has 
appreciably  increased  the  fertility  of  the  soil  of  Limagne — 
each  hectare  of  that  region  being  estimated  to  receive  1000 
kilos  of  dust  per  annum.  But  if  wind  in  some  cases  adds 
to  the  growth  of  soil  and  influences  its  character,  it  not 
infrequently  operates  adversely.  The  plateau  of  the  Karst, 
between  Carniola  and  I  stria,  for  example,  is  practically  devoid 
•of  soil — the  strong  winds  constantly  sweeping  it  away  from 
all  tracts  which  are  not  protected  by  forests,  or  sheltered 
by  the  configuration  of  the  ground.  In  like  manner,  the 
mountains  of  Provence  are  denuded  of  soil  by  the  mistral. 

Having  recognised  that  all  soils  consist  of  disintegrated 
rock  materials — derived  either  from  immediately  subjacent 
solid  rocks  or  from  more  or  less  incoherent  accumulations, 
under  which  the  latter  are  often  concealed,  writers  on  agri- 
culture have  classified  soils  as  Sedentary  and  Travelled,  or 
Transported.  The  terms  are  not  strictly  appropriate,  but 
they  may  serve  their  purpose  so  long  as  we  understand 
them  to  have  reference  to  the  nature  and  source  of  the 
materials  from  which  the  soils  have  been  derived.  It  might 
obviate  confusion,  however,  if  we  substituted  the  term  bed- 
rock soil  for  sedentary  soil,  and  adopted  the  term  drift  soil 
employed  by  our  Geological  Survey  in  place  of  travelled  or 
transported  soil.  We  should  thus  have  two  tolerably  well- 
defined  classes  of  soil — one  including  soils  derived  directly 
from  the  bed-rock,  and  the  other  embracing  every  soil 
formed  upon  the  surface  of  unconsolidated  "  superficial 
formations  "  of  all  kinds — whether  glacial,  alluvial,  or  aeolian. 

i.  BED-ROCK  SOILS 

Under  ordinary  conditions  the  soil-cap  covering  the  bed- 
rock shows  the  following  succession  : — 

(a)  Vegetable  Soil  or  Soil  Proper. — A  layer  of  variable  thick- 
ness, but  seldom  thinner  than  two  or  three  inches,  or  thicker 


SOILS  AND  SUBSOILS  377 

than  nine  or  a  dozen  inches.  Owing  to  the  presence  of  organic 
matter,  it  is  dark  in  colour.  It  may  consist  of  fine-grained, 
or  relatively  coarse-grained  materials,  or  of  a  mixture  of 
both — its  general  character  being  necessarily  determined  by 
that  of  the  underlying  subsoil  and  bed-rock.  Usually  it  is 
coarser  in  texture  than  the  subsoil  into  which  it  gradually 
passes. 

(b)  Subsoil. — An  earthy  accumulation  of  quite  indetermi- 
nate character  and  thickness,  but  commonly  finer  grained  and 
lighter   in   colour   than   the   vegetable   soil.      Fragments   of 
the  bed-rock  are  usually  scattered  more  or  less  abundantly 
through  the  subsoil,  but  are  most  plentiful  towards  the  bottom 
of  the  stratum,  where  they  often  form  a  kind  of  rough  rock- 
rubble.     The  subsoil  proper  contains  no  organic  matter. 

(c)  Bed-rock. — Just  as  the  soil  passes  down  gradually  into 
the  subsoil,  so  it  is  often  hard  to  say  where  subsoil  ends  and 
"  living-rock  "  begins.     The  upper  part  of  the  latter  is  often 
much  fissured,  earthy  matter  filling  the  cracks  until,  as  these 
are  followed  downwards,  they  close  up. 

The  character  of  the  disintegrated  materials  constituting 
soil  and  subsoil  naturally  depends  mainly  upon  that  of  the 
bed-rock.  Should  the  latter  be  made  up  of  relatively 
insoluble  ingredients — say  siliceous  sandstone,  quartzite, 
serpentine,  clay-slate  or  other  argillaceous  rock — the  soil-cap 
will  differ  but  slightly  in  character  from  the  bed-rock ;  it  will 
consist  simply  of  disaggregated  rock-material  which  has 
undergone  little  or  no  chemical  alteration.  In  such  cases,  the 
soil-cap  is  usually  thin  and  meagre.  On  the  other  hand,  if 
the  bed-rock  be  granite  or  any  other  highly  felspathic  rock,  it 
is  generally  more  or  less  deeply  decomposed.  The  rock- 
fragments  and  particles  of  the  soil  and  subsoil  are  likewise 
highly  altered,  the  subsoil  sometimes  attaining  a  thickness  of 
a  hundred  feet  or  more. 

As  disintegration  and  alteration  are  continually  in  pro- 
gress, the  subsoil  may  be  said  to  be  always  gaining  on  the 
bed-rock,  just  as  the  soil  continues  to  grow  at  the  expense  of 
the  subsoil.  The  soil  itself,  however,  does  not  necessarily 
increase  in  thickness,  for,  owing  to  the  action  of  rain  and 
wind,  its  surface  is  gradually  lowered,  the  finer  particles 
tending  to  be  washed  down  or  blown  away.  For  the  same 


378  STRUCTURAL  AND  FIELD  GEOLOGY 

reason,  coarser  grained  materials  become  concentrated  in  the 
soil,  which  thus  tends  to  acquire  a  coarser  and  more  open  texture 
than  the  subsoil,  more  especially  under  moist  climatic  condi- 
tions. The  rate  at  which  a  soil  wastes  away  varies  indefinitely. 
Where  the  ground  is  flat  and  thickly  clothed  with  vegetation, 
there  may  be  little  waste,  while,  owing  to  the  action  of  worms 
continually  bringing  up  fine-grained  materials  to  the  surface, 
the  soil  may  come  to  show  a  finer  texture  than  the  subsoil. 
Other  things  being  equal,  however,  surface-waste  naturally 
increases  with  the  slope  of  the  ground,  and  is  greater  when 
the  soil  is  bare  than  when  it  is  well  carpeted  with  verdure. 
As  absolutely  flat  ground  can  hardly  be  said  to  exist,  surface- 
waste  is  everywhere  in  progress,  on  steep  and  gentle  slopes 
alike.  Slowly  or  more  rapidly,  as  the  case  may  be,  dis- 
integrated rock-material  is  continually  being  urged  forward, 
and  eventually  finds  its  way  into  brooks  and  rivers.  In  this 
way  the  whole  surface  of  the  land  is  gradually  lowered. 
How  effective  such  action  has  been,  may  be  illustrated  by  the 
occurrence  upon  plateaus  and  flat  hill-tops,  of  rock-fragments 
derived  from  thick  formations  which  formerly  overspread 
those  regions,  but  have  now  entirely  disappeared.  As  an 
example,  we  may  cite  the  "  grey- wethers  "  or  "  sarcen-stones  " 
that  often  occur  in  the  soils  of  the  Chalk  Downs.  These 
fragments  of  siliceous  sandstone  are  the  relics  of  certain 
Tertiary  deposits,  which  at  one  time  covered  wide  areas  in 
southern  and  south-eastern  England.  During  the  slow 
growth  and  waste  of  the  soil-cap  the  Tertiary  deposits 
referred  to  have  been  gradually  but  persistently  removed, 
the  "  sarcen-stones "  (owing  to  their  size  and  their  insoluble 
character)  being  the  only  recognisable  relics  left  behind.  In 
this  way,  notwithstanding  the  persistence  of  a  soil-cap 
through  long  geological  ages,  the  whole  surface  of  a  country 
has  nevertheless  been  lowered  for  many  hundreds  of  feet. 

The  great  variety  of  bed-rock  soils  may  be  illustrated  by 
a  short  description  of  those  met  with  on  certain  well-known 
types  of  rock. 

Soils  from  Igneous  Rocks.— The  soils  derived  from  the 
disintegration  of  igneous  rocks  necessarily  vary  in  character. 
It  will  suffice,  however,  to  cite  a  few  typical  examples. 

Granite, — The  weathering  of  this  rock  has  already  been 


SOILS  AND  SUBSOILS  379 

referred  to,  and  we  have  learned  that  of  its  three  constituents 
quartz  is  practically  insoluble,  while  the  felspar  and  its  ferro- 
magnesian  associate  (mica  or  hornblende)  are  more  or  less 
readily  broken  up  and  resolved  into  kaolin  and  certain 
alkalies  and  alkaline  earths  which  tend  to  be  washed  away 
as  bi-carbonates  in  solution.  Under  favourable  conditions, 
therefore,  the  subsoil  overlying  granite  consists  of  an 
aggregate  of  larger  and  smaller  roughly  rounded  or  sub- 
angular  fragments,  in  all  of  which  the  felspar  is  more  or  less 
strongly  kaolinised.  These  fragments  are  set  in  a  gritty  clay- 
like  earth,  usually  reddish,  brownish,  or  yellowish  in  colour, 
through  which  non-elastic  scales  of  bleached  mica  may  be 
plentifully  scattered.  Although  the  soluble  carbonates  tend 
to  be  leached  out,  yet  a  larger  or  smaller  proportion  is  left 
behind,  for  the  gradual  decay  of  the  rock-fragments  and 
particles  is  continually  setting  free  fresh  supplies.  The 
vegetable  soil  does  not  differ  essentially  from  the  subsoil ; 
it  is  a  gritty  clay,  often  stonier  than  the  latter,  but  more 
deficient,  as  a  rule,  in  soluble  ingredients,  and  showing  few 
or  no  scales  of  mica.  • 

Since  granites  vary  in  character,  their  soil-caps  are  not 
uniformly  alike.  Very  coarse  varieties  necessarily  yield 
stonier  soils  than  the  finer  grained  kinds ;  while  the  resulting 
clays  often  differ  much  in  the  proportion  of  soluble  materials. 
The  soil  derived  from  granites  containing  hornblende  and  a 
notable  quantity  of  apatite,  may  be  expected  to  be  charged 
to  some  extent,  not  only  with  potash  and  lime,  but  phosphoric 
acid.  Much,  however,  depends  upon  the  position  occupied  by 
the  bed-rock.  In  the  low  grounds  of  non-glaciated  regions, 
granite  is  often  decomposed  to  a  very  great  depth,  and  may 
give  a  soil  capable  of  high  cultivation.  But,  in  our  own 
country,  the  rock  usually  occurs  at  mountainous  elevations, 
where  the  conditions  for  the  formation  of  a  persistent  soil-cap 
are  not  favourable.  Wind,  rain,  and  melting  snow,  and  the 
steep  gradients  of  the  surface,  all  conspire  to  prevent  the 
accumulation  of  disintegrated  rock-materials.  The  hill-slopes 
are  covered  with  sheets  of  grit  and  rough  gravel  ( =  quartz) ; 
over  the  low  grounds  the  clay  may  here  and  there  accumulate, 
but  the  soluble  materials  are,  for  the  most  part,  removed. 

Granite  may  be  taken  as  the  type  of  the  acidic  felspathic 


380  STRUCTURAL  AND  FIELD  GEOLOGY 

rocks.  Quartz-porphyries  and  rhyolites  yield  soils  of  much 
the  same  character — they  are  essentially  clays  with  a  larger 
or  smaller  percentage  of  sand  (quartz)  and  not  infrequently 
with  a  notable  proportion  of  potash,  magnesia,  and  lime. 
But,  just  as  with  granite,  the  character  of  the  soil-cap  is 
largely  determined  by  the  configuration  of  the  ground  and 
climatic  conditions.  In  short,  the  soil-cap,  according  to 
circumstances,  may  be  a  fine  sandy  clay,  or  a  mere  rubble  of 
sand,  grit,  and  rock-fragments. 

Basalt. — As  granite  is  the  type  of  the  acidic  igneous  rocks, 
so  basalt  may  be  taken  as  representative  of  the  basic  series. 
The  essential  constituents  of  this  rock,  it  will  be  remembered, 
are  felspar  and  augite  (usually  with  olivine),  and  generally  a 
considerable  proportion  of  magnetite  (often  accompanied  by 
ilmenite).  The  rock  commonly  weathers  to  some  depth, 
becoming  so  disintegrated  that  it  may  be  readily  dug  with  a 
spade.  The  resulting  soil  is  a  dark-coloured  loam — the  more 
notable  constituents  of  which  are  clay,  fine  sand,  iron-oxides, 
and  varying  proportions  of  the  carbonates  of  lime,  potash, 
magnesia,  together  often  -with  traces  of  phosphoric  acid, 
derived  doubtless  from  the  decomposition  of  apatite — a 
common  accessory  mineral  in  basalt  as  in  many  other  igneous 
rocks.  Where  the  surface  conditions  are  favourable,  basalt 
always  yields  rich  soils  of  this  character. 

Diorite,  Porphyrite,  etc. — That  large  class  of  igneous  rocks, 
the  silica  percentage  of  which  is  less  than  that  of  the  granites, 
quartz-porphyries,  etc.,  but  larger  than  that  of  the  basalts  and 
their  associates,  yield  soils  of  an  intermediate  character, 
which  would  be  classed  rather  as  loams  than  clays,  and  are 
often  highly  fertile.  The  diorites  and  porphyrites  are 
essentially  compounds  of  soda-lime  felspar  with  various 
ferromagnesian  minerals,  such  as  hornblende,  augite,  biotite, 
etc.  The  rocks  do  not,  as  a  rule,  weather  so  readily  as 
basalt,  but  this  is  not  always  the  case,  for  now  and  again 
their  decomposed  crusts  and  debris  can  hardly  be  distinguished 
from  disintegrated  and  decomposed  basalt.  Generally, 
1  however,  the  soil  derived  from  these  rocks  of  intermediate 
composition  contains  a  less  percentage  of  iron-oxide  than 
basalt-soil,  and  is  usually  more  clay-like.  The  subsoils,  as  one 
might  have  expected,  are  rich  in  lime  derived  chiefly  from  the 


SOILS  AND  SUBSOILS  381 

felspar,  but  also  to  some  extent  from  the  ferro-magnesian 
constituents.  Other  intermediate  rocks,,  as  syenite,  trachyte, 
phonolite,  give  subsoils  that  are  richer  in  potash  than  lime. 

Upon  the  whole,  then,  we  arrive  at  the  conclusion  that 
excellent  soils  may  be  derived  from  the  decomposition  of 
igneous  rocks  generally,  some  of  them  so  argillaceous  as  to 
be  properly  designated  clay-soils,  others  of  a  fine  loamy 
character,  and  yet  others  of  intermediate  character— but  all 
under  favourable  conditions,  being  capable  of  high  cultiva- 
tion. The  colour  'of  the  soils  is  lighter  or  darker,  according 
as  the  parent  rocks  are  poor  or  rich  in  iron-oxides.  Prob- 
ably, the  most  fertile  soils  are  those  yielded  by  the  basic 
rocks  (basalt,  etc.),  and  some  of  the  intermediate  rocks 
(diorites  and  porphyrites),  forming,  as  they  do,  dark  loams, 
rich  in  the  soluble  ingredients  required  for  the  growth  of 
plants. 

Soils  from  Metamorphic  Rocks.— The  weathering  of 
certain  metamorphic  rocks  results  in  the  formation  of  quite 
as  deep  and  good  soils  as  are  yielded  by  igneous  rocks 
generally.  On  the  other  hand,  many  of  the  schists  and  their 
associates  supply  only  meagre  barren  soils.  It  will  suffice  for 
our  purpose  to  note  one  or  two  examples. 

Gneiss. — As  this  rock  consists  of  the  same  mineral  con- 
stituents as  granite,  it  weathers  much  in  the  same  way,  and 
the  resulting  soil  is  similar — a  gritty  clay,  which,  according  to 
the  physical  conditions,  may  or  may  not  be  fertile.  At  high 
elevations  the  soil  is  either  a  mere  rubble  of  grit  and  stones 
or  a  thin  clay,  from  which  the  soluble  constituents  have 
usually  been  removed.  Under  more  favourable  conditions, 
as  regards  elevation  and  climate,  the  same  rock  may  be 
covered  with  a  deep  and  fertile  soil-cap. 

Mica-schist. — This  rock,  composed  of  quartz  and  mica  in 
variable  proportions,  often  yields  a  good  loamy  soil,  which  in 
favourable  positions  would  be  highly  esteemed  by  agriculturists. 
Unfortunately,  in  these  islands  it  usually  occurs  at  consider- 
able elevations,  where  climatic  conditions  do  not  favour 
cultivation.  Nevertheless,  the  fertility  of  the  soil  is  evidenced 
by -the  character  of  the  trees  it  supports— the  coniferous 
forests  grown  upon  the  mica-schists  of  the  Scottish  Highlands 
being  much  superior  in  every  respect  to  those  which  struggle 


382  STRUCTURAL  AND  FIELD  GEOLOGY 

for  existence  on  the  meagre  gritty  clay-soils  derived   from 
the  granites  and  gneisses  of  the  same  region. 

Amphibolites. — These  rocks  are  essentially  aggregates  of 
amphibole  (hornblende,  actinolite),  but  many  other  minerals 
are  often  present  They  yield  dark  loamy  soils  of  excellent 
quality,  quite  similar  in  character  to  those  of  the  diorites  and 
basalts.  The  rocks,  however,  do  not  occupy  large  areas  in 
Britain,  and  are  practically  confined  to  our  mountain  regions, 
where  the  conditions  are  unsuitable  for  agricultural  purposes. 

We  have  now  mentioned  some  of  the  metamorphic  rocks 
which  naturally  tend  to  yield  good  soils.  There  are  a  con- 
siderable number  of  the  same  group  of  rocks,  however,  which 
from  their  mineralogical  composition  could  not  be  expected 
to  supply  fertile  soils.  Amongst  these  may  be  mentioned 
clay-slate,  over  which  the  soil  is  usually  a  cold,  wet,  sterile 
clay.  Now  and  again,  however,  owing  chiefly  to  the  presence 
in  the  slate  of  felspathic  and  micaceous  ingredients,  the  soil 
may  be  of  somewhat  better  quality.  Another  very  unfavour- 
able rock  is  quartzite,  the  thin  soil  formed  upon  which 
consists  chiefly  of  chips,  splinters,  and  grains  of  the  rock 
held  together  sometimes  by  a  meagre  ferruginous  sand. 
Serpentine,  composed  of  a  somewhat  intractable  or  relatively 
insoluble  hydrous  magnesian  silicate,  is  not  more  favourable 
to  the  production  of  soil  than  quartzite,  the  thin  soil  yielded 
by  it  being  notable  for  its  infertility. 

Between  these  relatively  barren  soils  and  the  good  soils 
which,  under  favourable  conditions,  tend  to  form  upon  gneiss, 
mica-schist,  and  amphibolites,  there  are  soils  of  intermediate 
character  met  with  in  many  regions  of  metamorphic  rocks ; 
such  as  those  that  occur  above  granulite,  marble,  chlorite- 
schist,  etc.  The  soils  in  question  necessarily  vary  much  in 
character,  for  the  mineralogical  composition  of  the  rocks 
themselves  is  by  no  means  uniform.  The  soil  overly  ing- 
marble  is  usually  a  clay  (coloured  red,  brown,  or  yellowish, 
from  the  presence  of  iron-oxide),  which  may  contain  little  or 
no  trace  of  calcium  carbonate.  Marble,  however,  is  not 
infrequently  charged  with  many  "  new  "  or  "  contact-minerals," 
such  as  amphiboles  and  micas,  from  the  gradual  decomposi- 
tion of  which  in  subsoil  and  soil  carbonates  of  lime  and 
magnesia  may  be  derived.  Granulite,  on  the  other  hand, 


SOILS  AND  SUBSOILS  383 

yields  a  soil  comparable  to  that  derived  from  the  decom- 
position of  certain  gneisses  and  mica-schists.  The  soils 
formed  upon  chlorite-schist  are  commonly  thin,  gritty  clays, 
often  somewhat  dark  in  colour,  but  relatively  infertile,  although 
hardly  so  barren  as  the  soils  derived  from  serpentine,  quartzite, 
or  clay-slate. 

Soils  from  Derivative  Rocks. — These  rocks  consist,  for 
the  most  part,  of  arenaceous  and  argillaceous  materials ; 
amongst  them,  however,  are  included  many  important  cal- 
careous rocks.  No  doubt  there  are  numerous  other  kinds  of 
derivative  rocks,  but  inasmuch  as  the  outcrops  of  these 
occupy  very  limited  areas,  they  may  be  here  disregarded. 
Sandstones,  shales,  and  limestones  are  by  far  the  most  widely- 
spread  of  derivative  formations. 

Arenaceous  Rocks. — The  large  majority  of  these  rocks 
are  quartzose,  and  they  tend  therefore  to  yield  light  soils, 
which  are  often  not  sufficiently  retentive.  But  they  show 
great  differences  in  this  respect,  many  containing  larger  or 
smaller  percentages  of  argillaceous  matter,  and  giving  rise 
to  loamy  soils  of  excellent  quality.  Some  white  sandstones 
consist  almost  exclusively  of  quartz-grains,  and  owe  their 
induration  to  compression  alone.  The  soil  formed  upon  a 
rock  of  this  kind,  it  need  hardly  be  said,  will  be  a  barren 
sand,  incapable  of  tillage.  Many  sandstones,  however,  owe 
their  induration  to  some  cementing  material  that  binds  the 
grains  together.  The  cement  may  be  calcium  carbonate, 
iron-oxide,  argillaceous  matter,  or  other  substance.  When 
such  rocks  are  weathered,  therefore,  the  nature  of  the  cement- 
ing material  necessarily  affects  the  character  of  the  soil. 
Again,  it  may  be  noted  that,  although  quartz  is  the  dominant 
ingredient  of  most  sandstones,  yet  many  other  constituents 
are  sometimes  present.  Thus  sandstones  immediately  derived 
from  the  breaking-up  of  an  igneous  rock  may  consist  to  no 
small  extent  of  felspar,  mica,  and  other  minerals  in  a  more 
or  less  altered  condition.  Few  sandstones,  indeed,  do  not 
contain  scales  of  mica,  which  are  not  infrequently  so  abundant 
as  to  impart  a  fissile  structure  to  the  rock.  However  plentiful 
these  minerals  may  be  in  a  sandstone  itself,  they  are  not 
often  conspicuous  in  the  overlying  subsoil,  and  are  usually 
completely  wanting  in  the  vegetable  soil.  It  is  obvious, 


384  STRUCTURAL  AND  FIELD  GEOLOGY 

therefore,  that  they  must  become  decomposed,  and  that  their 
soluble  alkalies  and  alkaline  earths  are  available  for  the 
support  of  plant  life.  Some  sandstones  contain  so  much 
argillaceous  matter  that  their  weathered  materials  form  clay- 
like  rather  than  loamy  soils.  Such  is  usually  the  case  with 
the  palaeozoic  greywackes,  which  are  only  much  indurated 
argillaceous  sandstones.  The  soils  they  yield  are  usually  cold, 
retentive  clays.  Owing  to  the  fact  that  these  rocks  occur, 
as  a  rule,  in  high-lying  districts,  their  soils  are  seldom  tilled. 
In  low-lying  districts,  however,  soils  of  the  same  origin,  when 
they  can  be  well-drained,  are  cultivated  with  success.  Grey- 
wacke,  it  may  be  added,  often  contains  much  felspathic 
material,  which,  on  decomposing,  supplies  alkalies  and  alka- 
line earths. 

Argillaceous  Rocks. — These  naturally  give  clay-soils,  but, 
owing  to  the  variable  character  of  the  rocks,  there  are  as 
many  differences  among  clay-soils  as  among  arenaceous  soils. 
Some  argillaceous  strata  contain  so  much  sand,  that  the  soil 
resulting  from  their  disintegration  might  be  classed  among 
the  loams  or  transition  soils,  being  neither  clays  nor  sands. 
Not  a  few  clay  rocks  consist  almost  entirely  of  clay  and 
quartz  in  the  very  finest  state  of  division,  all  soluble  ingredients 
being  practically  wanting.  The  soils  formed  upon  these  are, 
it  need  hardly  be  said,  very  infertile.  Certain  argillaceous 
rocks,  on  the  other  hand,  may  be  largely  charged  with 
calcareous  matter,  and  would  be  then  termed  marls,  some 
of  which  yield  excellent  soils.  It  may  be  noted  here,  how- 
ever, that  the  term  marl,  as  used  by  some  geologists,  is 
misleading.  Many  of  the  so-called  "  marls  "  of  the  Old  Red 
Sandstone,  the  Permian,  and  the  Triassic  systems  contain 
no  carbonate  of  lime,  but  are  simply  clays  with  a  larger  or 
smaller  percentage  of  sand.  As  they  occur  interbedded  with 
sandstones,  the  overlying  soils  usually  assume  the  character 
of  a  "  strong  loam,"  forming  what  is  one  of  the  most  fertile 
soils  met  with  in  these  islands.  Good  examples  are  furnished 
by  the  famous  "red  soils"  of  East  Lothian,  Wales,  and 
Cornwall,  all  of  which  overlie  rocks  of  Old  Red  Sandstone 
age,  and  the  somewhat  similar  soils  ("  red  ground  ")  yielded 
by  the  Triassic  Keuper  Marl  of  Cheshire  and  the  Mid- 
lands. Clay-rocks  in  general,  however,  tend  to  give  heavy 


SOILS  AND  SUBSOILS  385 

clay-soils,   which   are   nowadays   seldom   tilled,   but  kept  in 
grass. 

Calcareous  Rocks.— An  absolutely  pure  limestone  would 
be  incapable  of  yielding  a  soil.  All  limestones,  however,  do 
contain  insoluble  impurities,  such  as  sand  and  clay,  and  the 
soils  derived  from  them  are  thus  usually  either  loams  or  clays. 
Good  examples  of  such  soils  are  those  met  with  in  the  Chalk 
districts  of  England.  They  are  usually  reddish  or  brown  in 
colour,  and  vary  in  character  from  stiff,  retentive  clays  to 
calcareous  loams.  The  soil-caps  of  those  regions  are 
naturally  thickest  in  the  valleys — the  tops  and  steeper 
slopes  of  the  hills  showing  little  or  no  soil  at  all.  In  some 
places,  however,  the  hills  are  capped  with  sheets  of  flint- 
gravel — the  flints  having  been  derived  from  the  gradual 
dissolution  of  the  chalk  in  which  they  were  formerly  embedded. 
Limestones  all  the  world  over  yield  similar  reddish,  yellowish, 
or  brownish  clays  and  loams — the  colour  being  due  to  the 
presence  of  iron-oxides.  The  famous  terra  rossa  of  Southern 
Europe  is  a  well-known  example.  As  most  limestones  are 
traversed  by  joints  which  have  been  widened  by  the  action 
of  acidulated  water,  much  of  the  insoluble  red  earth  formed 
at  the  surface  is  washed  by  rain,  and,  in  some  cases,  by 
melting  snow,  into  these  open  fissures.  Limestone  regions, 
therefore,  especially  when  relatively  high,  are  apt  to  show  a 
rocky  surface,  sparingly  sprinkled  with  a  thin  clay-like  or 
loamy  soil. 

-jf.' _  2.  DRIFT  SOILS 

Under  this  head,  as  already  explained,  may  be  grouped 
all  soils  which  do  not  owe  their  origin  to  the  direct  disin- 
tegration of  the  bed-rock,  but  are  the  modified  upper 
portions  of  glacial,  alluvial,  and  aeolian  accumulations  of 
every  kind.  The  materials  of  which  they  are  composed 
have  been  transported  for  shorter  or  greater  distances. 

Glacial  Soils — These  soils  are  usually  somewhat  tena- 
ceous  clays,  but  vary  considerably  in  character.  They 
overlie  accumulations  of  glacial  origin,  which  may  consist 
either  of  stony  or  essentially  stoneless  clays — the  latter 
being  usually  laminated,  while  the  former  are  commonly 
amorphous. 

2  B 


386  STRUCTURAL  AND  FIELD  GEOLOGY 

Boulder-clay  or  Till  is  the  general  term  applied  to  the 
stony  clays.  These  clays,  being  of  subglacial  origin,  consist 
almost  exclusively  of  crushed  and  comminuted  rock.  In 
other  words,  they  consist  of  unweathered  materials — differing 
in  this  respect  from  all  clays  of  truly  sedimentary  origin.  As 
a  rule,  these  stony  or  boulder-clays  are  of  a  highly  imperme- 
able character,  and  consequently,  the  soil  formed  upon  them 
is  usually  thin.  The  subsoil  is  also  thin,  and  the  materials  of 
which  it  is  composed  show  commonly  little  trace  of  weathering 
— the  most  notable  change  being  the  partial  oxidation  of 
ferruginous  constituents.  Thus,  a  blue-coloured  boulder-clay 
may  graduate  upwards  into  a  yellowish  or  brownish  clay,  two 
or  three  feet  in  thickness,  overlaid  by  a  few  inches  of  a  more 
or  less  stony,  tenaceous  clay-soil.  It  may  be  said  in  general 
terms  that  the  colour  and  composition  of  boulder-clay  are 
determined  by  the  nature  of  the  bed-rock  upon  which  or  near 
to  which  it  lies.  Thus,  in  a  district  where  red  sandstones 
predominate,  the  overlying  boulder-clay  is  usually  reddish 
and  more  or  less  arenaceous :  where  Carboniferous  rocks 
prevail  (light  coloured  sandstones,  black  shales,  fireclay,  iron- 
stones, coal,  limestone,  etc.),  the  till  is  dull  bluish-grey  in 
colour,  and  often  exceedingly  tenaceous  :  when  the  dominant 
country-rock  is  chalk,  the  boulder-clay  forms  a  dirty  greyish- 
white  marl.  A  good  geological  map,  therefore,  although  it 
may  not  show  the  superficial  formations  of  a  country,  is 
nevertheless  often  a  reliable  index  to  the  average  character 
of  the  boulder-clays.  The  general  local  character  of  the  till, 
however,  does  not  hold  good  throughout.  If  we  follow  the 
direction  of  ice-flow  in  a  country  like  Scotland,  we  soon 
discover  that  there  are  limits  to  the  local  character  of  the  till. 
Coming  down  the  valley  of  the  Tweed,  for  example,  from  the 
heart  of  the  Silurian  Uplands  to  the  low  grounds  near 
Melrose,  we  find  that,  so  long  as  we  are  in  the  region  of 
greywacke  and  shale,  the  till  is  a  fawn-coloured,  tenaceous, 
gritty  clay,  crowded  with  angular  and  subangular  fragments 
of  the  country-rock.  As  we  leave  the  Silurian  strata  and 
enter  the  region  of  Old  Red  Sandstone,  the  till  continues  to 
be  composed  of  the  debris  of  greywacke  and  shale,  although 
here  and  there  fragments  of  the  underlying  red  sandstones 
begin  to  appear.  Continuing  down  the  valley,  red  sandstone 


SOILS  AND  SUBSOILS  387 

boulders  become  more  and  more  numerous,  while  the  colour 
and  texture  of  the  clay  gradually  change,  as  crushed  and 
comminuted  sandstone  enters  more  and  more  largely  into  its 
composition.  The  majority  of  the  stones  and  boulders,  how- 
ever, are  still  of  Silurian  parentage — doubtless  due  to  the 
fact  of  their  superior  durability — the  red  sandstones  being 
much  more  readily  pulverised.  The  till  continues  to  present 
much  the  same  appearance  after  the  region  of  red  sandstone 
has  been  left  behind,  but  gradually  it  loses  its  pronounced 
red  colour  as  Kelso  is  approached,  while  fragments  of  certain 
igneous  rocks,  which  crop  to  the  surface  above  that  town, 
begin  to  abound.  From  Kelso  to  the  sea  the  prevalent  rocks 
are  brown,  reddish,  grey,  and  white  sandstones,  sandy  shales, 
marls,  etc.  The  overlying  till,  therefore,  is  a  somewhat 
arenaceous  clay,  light  brown  as  a  rule,  but  here  and  there 
with  a  reddish  tinge.  The  bulk  of  the  finer  grained  materials 
of  this  till  are  of  local  origin,  but  the  most  conspicuous  stones 
and  boulders  are  still  greywacke  commingled  with  abundant 
fragments  of  the  "  Kelso  trap-rocks,"  and  other  igneous  rocks 
traversed  by  the  old  ice-sheet  Similar  phenomena  are 
encountered  everywhere  in  regions  where  boulder-clay  occurs. 
While  it  is  true,  therefore,  that  this  accumulation  has  a  more 
or  less  local  character — and  this  is  especially  the  case  writh  its 
finer  grained  materials — yet  it  must  be  remembered  that  the 
till  formed  in  one  place  tended  to  travel  forward  with  the  ice. 
In  this  way  boulder-clay  often  came  to  be  deposited  upon 
bed-rock  of  a  very  different  character  from  that  of  the  region 
where  it  was  actually  formed.  No  small  proportion  of  the 
stones,  and  even  of  the  gritty  and  clay-like  material  of  the 
till  that  covers  the  low  grounds  of  a  country,  is  often  of  more 
or  less  distant  derivation. 

It  will  now  be  readily  understood  that  the  soils  yielded 
by  boulder-clay  arc  of  very  unequal  character  and  value. 
The  dark  lead-coloured  tenaceous  till  met  with  in  many 
Carboniferous  tracts  gives  a  most  ungrateful  soil — a  thin, 
cold,  unctuous,  sticky  clay  in  wet  weather ;  and  in  drought, 
hard  and  unyielding.  In  other  places  within  the  same 
geological  area,  the  till  has  proved  more  kindly — owing 
chiefly  to  a  larger  proportion  of  comminuted  sandstone, 
limestone,  and  igneous  rocks.  The  red  and  brown  coloured 


388  STRUCTURAL  AND  FIELD  GEOLOGY 

boulder-clay  soils,  however,  are  upon  the  whole  the  best. 
These  consist  mainly  of  pulverised  red  sandstone,  and  form 
strong  loams,  rather  than  clays.  Chalky  boulder-clays,  com- 
posed chiefly  of  pulverised  chalk,  yield  clay-soils  from  which 
the  calcareous  constituents  have  sometimes  been  largely 
removed. 

The  agricultural  treatment  of  soils  is  a  subject  on  which 
the  geologist  has  no  title  to  speak.  He  may,  however,  be 
allowed  to  point  out  the  danger  of  deep-ploughing  upon 
boulder-clay  soils  of  all  kinds.  Undisturbed  boulder-clay 
consists  almost  exclusively  of  unweathered  materials — its 
mineral  constituents  are  quite  unaltered,  and  it  is  therefore 
in  no  sense  of  the  term  a  subsoil.  It  plays  the  part,  in  fact, 
of  unweathered  "  bed-rock."  Owing  to  its  impervious 
character,  the  subsoil  and  soil  formed  upon  it  seldom  exceed 
a  foot  or  two  in  thickness.  Now  and  again,  as  in  sandstone 
regions,  it  is  somewhat  more  pervious,  and  covered,  there- 
fore, with  a  thicker  soil-cap.  In  some  boulder-clay  tracts, 
indeed,  the  arable  soil  considerably  exceeds  a  foot.  It  will 
be  found,  however,  that  these  thicker  soils  have  not  been 
derived  exclusively  from  the  boulder-clay  upon  which  they 
lie,  but  have  in  large  measure  been  washed  down  by  rain 
from  adjacent  slopes.  This  is  shown  not  only  by  the  unusual 
thickness  of  the  soil  in  question,  but  by  the  fact  that  it 
contains  relatively  few  stones,  while  the  much  thinner  soils 
of  the  neighbouring  banks  and  mounds  are  crowded  with 
stones,  the  tops  of  the  banks  being  not  infrequently  covered 
with  a  thick  sheet  of  course  shingle  and  boulders. 

Stoneless  Clays. — These  deposits  consist  usually  of  very 
fine  gutta-percha  clays.  They  are  generally  well  laminated, 
and  are  confined  in  these  islands  to  maritime  districts,  where 
they  seldom  occur  more  than  130  feet  above  the  sea-level. 
They  are  best  developed  in  the  lower  reaches  of  the  great 
valleys  of  Central  Scotland,  where  they  form  a  considerable 
proportion  of  the  Carse-lands  of  the  Tay,  the  Forth,  and 
the  Clyde.  Clays  of  precisely  the  same  character  are  met 
with  in  the  Newcastle  and  Durham  districts.  The  deposits 
have  occasionally  yielded  shells  of  Arctic  molluscs,  and  now 
and  again  an  isolated  stone  or  boulder  appears :  in  a  few 
places,  indeed,  small  and  large  erratics  are  of  quite  common 


SOILS  AND  SUBSOILS  389 

occurrence,  but  that  is  exceptional.  The  clays  are  obviously 
of  marine  origin,  deposited  at  a  time  when  Arctic  climatic 
conditions  obtained.  When  carefully  examined,  they  prove 
to  consist  for  the  most  part  of  minute  particles  of  rock  and 
mineral,  which,  as  a  rule,  are  as  fresh  and  unaltered  as  the 
similar  fine  ingredients  of  boulder-clay.  When  thoroughly 
washed  and  sifted,  the  clay  yields  an  exceedingly  fine  rock 
meal  or  flour,  of  which  the  most  abundant  constituent  is 
quartz.  True  clay  or  kaolin  is  of  subordinate  importance, 
but  appears  to  be  rather  more  abundant  than  in  most  true 
boulder-clays.  These  stoneless  clays,  therefore,  would  appear 
to  be  of  the  same  origin  as  the  former — they  are  the  result 
of  glacial  grinding,  and,  unlike  ordinary  alluvial  clays,  are 
not  the  product  of  the  mechanical  and  chemical  process  of 
weathering.  They  represent  the  fine  mud,  etc.,  swept  into 
our  estuaries  by  turbid  rivers  escaping  from  large  glaciers, 
and  too  short  a  time  elapsed  before  they  settled  down,  to 
allow  of  much  chemical  alteration.  It  need  hardly  be  said 
that  the  soils  met  with  upon  such  clays  are  peculiarly 
tenaceous,  except  where,  as  sometimes  happens,  thin  layers 
and  bands  of  sand  are  intercalated  in  the  upper  part  of 
the  deposits.  Deep-ploughing  upon  these  clays  is  obviously 
not  less  to  be  avoided  than  in  the  case  of  true  till. 

Alluvial  Soils. — Under  this  head  we  include  all  superficial 
deposits  consisting  of  disintegrated  and  weathered  rock- 
materials,  which  have  been  transported  and  spread  out  by 
water.  Some  of  these  formations  have  been  accumulated 
in  fresh  water,  others  have  been  laid  down  in  estuaries  and 
upon  the  sea  floor.  They  and  their  soil-caps  naturally  vary 
much  in  character.  The  coarser  deposits  consist  of  water- 
worn  shingle  and  gravel,  and  are  generally  barren,  owing  to 
the  rapidity  with  which  rain  is  absorbed.  Any  soil  formed 
at  the  surface  tends  in  this  way  to  disappear.  Now  and 
again,  however,  when  the  interstices  between  the  stones 
are  well  filled  with  grit  and  sand,  a  light  porous  soil  is 
formed.  Between  such  coarse  accumulations  and  the  finest 
deposits  of  mud  and  silt,  we  meet  with  all  gradations.  Of 
the  sands,  quartz  is  the  dominant  ingredient,  and  a  pure 
quartz-sand,  it  need  hardly  be  said,  cannot  furnish  a  good 
soil.  Many  sands,  however,  contain  larger  and  smaller 


390  STRUCTURAL  AND  FIELD  GEOLOGY 

percentages  of  clay,  and  may  form  loamy  soils  of  excellent 
quality.  From  loams  capable  of  high  cultivation,  we  pass 
on  to  clays,  many  of  which  are  tenaceous,  although  few 
alluvial  clays  have  the  tenacity  so  characteristic  of  the  tills 
and  stoneless  clays  of  glacial  origin.  Alluvial  clays  and 
muds  often  contain  much  organic  matter,  and  are  frequently 
rich  in  soluble  mineral  salts.  As  examples  of  alluvial  for- 
mations may  be  mentioned  the  flats  and  terraces  of  our 
river  valleys,  the  great  Carse-lands  of  Middle  Scotland,  the 
raised  beaches  of  our  maritime  tracts,  and  the  many  patches 
of  level  ground  which  indicate  the  sites  of  former  lakes. 
Under  the  same  head,  also,  we  may  include  the  fluvio-glacial 
heaps  and  sheets  of  gravel,  sand,  etc.,  shortly  described  in 
Chapter  XX.  Although  these  deposits  are  primarily  of  sub- 
glacial  derivation,  yet  their  materials  must  obviously  have 
been  more  or  less  altered  and  disintegrated  while  they  were 
being  transported  and  distributed.  Moreover,  their  highly 
permeable  nature  has  allowed  the  free  passage  of  rain,  so  that 
in  time  all  such  fluvio-glacial  gravels  and  sands  have  acquired 
much  the  same  character  as  the  similar  deposits  formed  by 
ordinary  river-action.*  From  an  agricultural  point  of  view, 
therefore,  they  may  be  included  under  one  and  the  same  head. 
In  fine,  it  will  be  understood  that  the  chief  distinction 
between  "alluvial-formations"  and  those  which  have  been 
described  under  the  head  of  "glacial  soils,"  is  simply  this, 
that  the  former  consist  essentially  of  "weathered"  rock- 
material,  while  the  latter  are  composed  almost  exclusively  of 
"  unaltered  "  mineral  matter — of  crushed,  pounded,  and  pul- 
verised rock,  which  had  previously  undergone  little  or  no 
chemical  alteration.  The  "  alluvial  formations,"  in  a  word, 
consist  of  disintegrated  rock-material,  and  are  true  "  subsoils." 
Glacial  clay-deposits,  on  the  other  hand,  are  not  "  subsoils," 
but,  properly  speaking,  unaltered  "  bed-rock." 

*  It  may  be  noted,  however,  that  the  stones  of  a  fluvio-glacial  gravel 
are  usually  fresher  than  those  of  modern  alluvial  deposits.  Their 
weathered  crusts  are  thinner,  and  they  are  sounder  internally.  This 
is  most  notable  in  the  case  of  basalt  and  similar  rocks.  When  the 
stones  of  a  modern  gravel-bed  have  been  derived  directly  from  till, 
however,  they  are  usually  quite  as  sound  as  those  occurring  in  a  fluvio- 
glaciardeposit. 


.      SOILS  AND  SUBSOILS  391 

Soils. — The  most  notable  aeolian   accumulations 
are  the  sand  dunes  of  maritime  districts  and  certain  inland 
areas.     As  the  dominant  ingredient  of  all  dunes  is  quartz, 
they  can  hardly  be  said  to  yield  a  soil.     Nevertheless,  certain 
sand-loving  plants  find  sustenance  upon  them,  and  succeed 
in  binding  the  loose  grains  together,  so  that  eventually  some 
amount  of  humus  is  accumulated,  and  a  thin  soil  is  formed. 
But  if  aeolian  sand  accumulations  yield  very  poor  soils,  such 
is  not   the  case  with   certain  other  wind-blown  formations. 
The  fine  dust  swept  by  the  wind  from  desiccated  regions,  and 
distributed    over  adjacent  tracts,  may  not  only  add  to  the 
fertility  of  soils,  but  under  certain  conditions  may  accumulate 
to  such  an  extent  as  to  conceal  all  bed-rock  and  native  soil- 
caps  over  extensive  areas.     There  can  be  little  doubt  that 
desert-dust   has   added  to   the   fertility  of  the   Nile  Valley, 
while,  according  to  Baron  Richthofen,  the  massive  sheets  of 
loess  which  cover  enormous  tracts  in   China  are  true  dust- 
deposits,  gradually  accumulated  by  the  winds  flowing  out- 
wards   from   the   desiccated    regions   of    Central    Asia.     In 
Europe,  a  similar  deposit  occurs  in  the  Rhine  Valley  and  in 
the  low  grounds  traversed  by  the  Danube,  while  the  extensive 
sheets  of  "  black-earth  "  forming  the  great  plains  of  Southern 
Russia  are  also  a  variety  of  loess.     The  origin  of  the  European 
loess  has  been  much  discussed,  but  it  seems  to  be  now  the 
general  opinion  that  the  materials  of  the  loess  were,  in  the  first 
place,  of  glacial  and  fluvio-glacial  origin.     The  fine  sand  and 
clay  that  resulted  from  glacial  grinding  appear  to  have  been 
introduced   to  the   low  grounds   of  Europe   largely   by   the 
flooded  rivers  and  inundations  of  the  Ice  Age.     It  is  thought, 
however,  that  wind   also  played   an    important   part   in  the 
transport  of  these  fine-grained  materials.     Northern  Europe 
covered  with  an  ice-sheet  must  have  formed  an  area  of  high- 
pressure,  from  which  strong   winds  would   flow  southwards. 
During  the  severe  winter  season,  all  the  streams  and  rivers 
would   be   reduced    in   volume,   and   wide   areas,   no   longer 
inundated,  would   be   exposed   to   the   disintegrating   action 
of  frost.      The    fine   fluvio-glacial   deposits   thus   pulverised 
would  be  ready  to  be  swept  up  by  strong  winds,  and  dis- 
tributed over  wide  areas  in  Central  and  South-eastern  Europe. 
Similarly,  the  fluvio-glacial  accumulations  of  the  low  grounds, 


392  STRUCTURAL  AND  FIELD  GEOLOGY 

dried  and  desiccated,  would  in  like  manner  tend  to  drift 
before  the  wind.  In  some  such  way,  deposits  originally  of 
glacial  and  fluvio-glacial  origin  have  been  rearranged  and 
redistributed  by  seolian  action.  This  conclusion  is  supported 
by  the  occurrence  in  the  loess  of  the  remains  of  a  true  Steppe- 
fauna — embracing  jerboas,  pouched  marmots,  tailless  hare, 
little  hamster  rat,  and  many  other  forms  which  are  the 
common  denizens  to-day  of  the  Steppes  of  Eastern  Russia 
and  Western  Siberia. 

Loess  may  be  described  as  a  fine  calcareous  loam,  consist- 
ing of  an  admixture  of  minute  particles  of  quartz  and  clay. 
The  percentage  of  calcium  carbonate  varies,  often  reaching 
or  even  exceeding  30  per  cent.  Its  light  red  or  yellowish 
colour  is  due  to  the  presence  of  iron-oxide.  Small  percentages 
of  magnesia,  potash,  soda,  and  phosphoric  acid  are  usually 
present.  Loess  thus  yields  an  excellent  soil,  the  regions 
covered  by  it  being  noted  for  their  great  fertility. 


CHAPTER  XXV 

GEOLOGICAL  STRUCTURE  AND  SURFACE  FEATURES 

Denudation  and  the  Evolution  of  Surface  Features.  Mountains  classified 
according  to  Structure  and  Origin  ;  Original  or  Tectonic  Mountains 
— their  Erosion  and  Transformation  ;  Subsequent  or  Relict  Moun- 
tains. Plains  and  Plateaus  of  Accumulation  and  Erosion.  Original 
or  Tectonic  and  Subsequent  or  Erosion  Valleys.  Basins. 

THE  student  of  structural  geology  soon  learns  to  recognise 
the  importance  of  denudation.  Almost  everywhere  he  is 
confronted  with  evidence  to  show  that  enormous  masses  of 
rock  have  been  removed  from  the  land.  He  is  prepared, 
therefore,  to  believe  that  erosion  must  have  played  an 
important  part  in  the  production  of  surface  features.  As  his 
observations  extend,  he  begins  to  realise  that  the  configura- 
tion or  shape  of  the  land  largely  depends  upon  the  lithological 
character  and  the  geological  structure  of  the  underlying  rock- 
masses.  So  constantly  does  this  remarkable  relationship 
appear,  that  it  cannot  be  merely  accidental.  In  regions  which 
have  been  highly  disturbed  within  late  geological  times,  it 
is  doubtless  true  that  the  larger  or  more  prominent  features 
of  the  land  are  the  direct  result  of  deformation.  We  see 
certain  great  mountain  chains,  the  geological  structure  of 
which  compels  us  to  believe  that  they  are  simply  wrinkles 
or  flexures  of  the  earth's  crust.  But  in  other  lands  which  have 
not  been  subject  to  crustal  deformation  for  a  prolonged  period  of 
time,  we  can  seldom  trace  any  coincidence  between  surface 
features  and  underground  structures.  The  mountains  do  not 
correspond  with  anticlinal  folds — the  valleys  do  not  lie  in 
synclinal  troughs.  The  relation  between  the  configuration  of 
the  ground  and  its  internal  structure  is  therefore  not  direct. 
In  a  word,  observation  has  conclusively  shown  that  in  all 

393 


394  STRUCTURAL  AND  FIELD  GEOLOGY 

highly  denuded  lands  the  forms  assumed  by  the  surface  have 
been  determined  mainly  by  the  character  of  the  rocks  and 
the  mode  of  their  arrangement  Hence  it  may  be  said,  in 
general  terms,  that  when  a  region  is  exposed  to  denudation, 
its  relatively  "  soft "  rocks  and  weak  structures  will  be  reduced 
more  rapidly  than  its  relatively  "  hard "  rocks  and  strong 
structures,  the  latter  coming  in  time  to  dominate  the  former 
in  a  more  and  more  pronounced  degree. 

To  sum  up,  we  may  conclude  that  many  prominent  features 
of  the  land  are  directly  due  to  deformation  of  the  crust,  and 
that  others  are  the  immediate  result  of  denudation  and 
accumulation,  while,  between  these  two  groups  of  land-forms, 
we  recognise  an  intermediate  type  in  the  production  of  which 
both  hypogene  and  epigene  forces  have  been  concerned.  A 
glance  at  the  structure  and  origin  of  the  leading  surface 
features  of  the  land  may  suffice  to  make  this  clear. 

i.  MOUNTAINS 

There  are  only  two  great  classes  of  mountains.  One  of 
these  comprises  every  height  which  owes  its  origin  either  (a) 
to  the  heaping-up  of  materials  at  the  surface  of  the  earth,  or 
(b)  to  subterranean  action,  which  has  resulted  in  the  deforma- 
tion of  the  earth's  crust.  These  two  groups  form  the  great 
class  of  Original  or  Tectonic  Mountains.  The  second  great 
class  includes  a  vast  variety  of  heights  of  quite  a  different 
origin.  They  have  not  been  built  up  by  accumulation,  nor 
upheaved  by  crustal  movements,  but  are  simply  the  surviving 
remnants  of  ancient  high  lands.  They  now  form  mountains 
because  the  land  masses  by  which  they  were  at  one  time 
encompassed  have  gradually  been  worn  down  and  removed. 
Lofty  plateaus,  for  example,  have  been  so  deeply  excavated 
by  superficial  action,  that  they  have  frequently  lost  their 
plateau  character  and  acquired  quite  a  mountainous  aspect. 
Mountains  of  this  kind  we  term  Subsequent  or  Relict. 

i.  ORIGINAL  OR  TECTONIC  MOUNTAINS. — These  include 
two  groups,  namely,  Accumulation  Mountains  and  Deforma- 
tion Mountains^  of  which  the  latter  is  by  far  the  more 
important. 

(a)  Accumulation  Mountains. — We  need  not  dwell  on 


STRUCTURE  AND  SURFACE  FEATURES  395 

the  phenomena  characteristic  of  this  group.  It  is  typically 
represented  by  volcanoes,  the  structure  and  origin  of  which 
are  familiar.  Volcanoes  may  occur  singly  or  in  irregular 
groups,  or  they  may  be  distributed  along  definite  lines  or 
zones.  They  vary  in  size  from  mere  monticles  up  to  vast 
cones.  Sometimes  they  appear  in  low-lying  regions,  at  other 
times  they  occur  upon  the  flanks  or  are  strung  along  the 
crests  of  lofty  mountain  ranges.  Built  up,  as  they  are,  of 
materials  ejected  from  below — of  molten  rock  alone,  or  of 
loose  stones,  dust,  and  ashes  alone,  or,  as  is  most  frequently 
the  case,  partly  of  one  and  partly  of  the  other — they  all  have 
essentially  the  same  structure.  They  consist  of  successive 
layers  of  variable  thickness,  sloping  outwards  in  all  directions 
from  the  centre  or  centres  of  eruption.  The  actual  form  of  a 
growing  cone  depends  very  largely  upon  the  nature  of  the 
materials  of  which  it  is  composed.  Thus  a  volcano  which 
emits  highly  fluid  lavas  does  not,  as  a  rule,  throw  out  loose 
materials  in  much  abundance.  Hence  such  volcanoes  have 
usually  the  form  of  more  or  less  depressed  cones — the  liquid 
lavas  flowing  away  and  spreading  out  rapidly,  while  the 
limited  supply  of  loose  ejecta  does  not  favour  rapid  growth  in 
the  vicinity  of  the  crater.  Viscous  lavas,  on  the  other  hand, 
do  not  flow  so  rapidly,  and  tend,  therefore,  to  coagulate  at  no 
great  distance  from  the  focus  of  eruption.  And  as  they  are 
generally  accompanied  by  abundant  discharges  of  loose 
ejecta,  the  resulting  cone  is  usually  more  or  less  abrupt. 
Hence,  in  the  case  of  active  volcanoes,  the  external  form  is  an 
expression  of  the  internal  or  geological  structure. 

Volcanoes  are  subject,  like  all  other  portions  of  the  land- 
surface,  to  the  modifying  influence  of  the  superficial  or 
epigene  agents  of  change.  While  in  a  state  of  activity  they 
are  worn  and  degraded  by  rain  and  torrents,  by  which  they 
are  often  deeply  scarred  and  furrowed.  But  such  ravages 
are  more  than  compensated  for  during  times  of  eruption  and 
accumulation.  When  volcanic  action  ceases,  however,  there 
can  be  no  such  compensation — degradation  then  proceeds 
apace.  The  more  or  less  steep  inclination  of  the  surface  and 
,the  weak  geological  structure  favour  the  action  of  the  epigene 
agents.  Gullies  and  ravines  are  rapidly  deepened  and 
widened ;  rock-falls  and  landslips  ever  and  anon  take  place ; 


396  STRUCTURAL  AND  FIELD  GEOLOGY 

till  sooner  or  later  the  symmetry  of  the  original  cone  dis- 
appears. Undermined  and  breached  in  every  direction,  the 
mountain  eventually  loses  the  distinctive  aspects  of  a  growing 
volcano.  After  a  prolonged  period  all  that  may  be  left  pro- 
jecting above  the  general  level  of  the  land  may  be  the  core 
or  plug  of  igneous  rock  which  cooled  and  consolidated  in  the 
funnel  or  pipe  of  eruption.  Volcanoes,  then,  may  be  looked 
upon  as  typical  "  accumulation  mountains,"  in  which,  while 
they  are  still  growing,  internal  structure  and  external  form 
coincide.  No  sooner  are  they  extinct,  however,  than  they 
begin  to  lose  their  characteristic  shape,  and  a  time  at  last 
arrives  when  there  ceases  to  be  any  correspondence  between 
structure  and  configuration. 

But  volcanoes  are  not  the  only  examples  of  "  accumulation 
mountains."  Materials  are  heaped  up  at  the  earth's  surface 
by  other  than  subterranean  action.  Thus  dunes  owe 
their  origin  to  the  action  of  wind,  while  moraines  are  built 
up  by  glaciers.  True,  neither  dunes  nor  moraines  attain  the 
dimensions  usually  indicated  by  the  term  mountain.  The 
term,  however,  is  rather  popular  than  scientific,  and  in  a 
scientific  classification  must  be  taken  to  include  not  only 
the  loftiest  elevations,  but  inconsiderable  hills  and  monticles  as 
well.  For  our  classification  is  based  essentially  on  geological 
structure  and  origin,  and  takes  no  note  of  the  relative 
dimensions  of  hills  and  mountains.  Amongst  "  accumulation 
mountains,"  therefore,  we  must  arrange  dunes,  moraines, 
and  all  other  hills  which  are  due  to  the  heaping-up  of 
materials  at  the  surface  by  natural  causes.  Many  of  these 
epigenetic  hills  are  doubtless  of  insignificant  size.  Dunes 
and  moraines,  for  example,  do  not  very  often  exceed  100 
feet,  but  now  and  again  the  former  attain  heights  of  400 
to  600  feet,  while  the  latter  not  infrequently  reach  greater 
heights,  sometimes  forming  hills  over  1000  feet  high. 

(U)  Deformation  Mountains.  —  Under  this  head  are 
included  all  mountains  which  owe  their  origin  to  deformation 
of  the  earth's  crust.  Three  types  are  recognised,  namely, 
Folded  Mountains  (due  essentially  to  folding  and  crumpling 
of  the  crust),  Dislocation  Mountains  (due  to  fracturing  and  dis- 
location of  the  crust),  and  Laccolith  Mountains  (due  to  bulging- 
up  of  the  crust  over  intrusive  masses  of  igneous  rock). 


STRUCTURE  AND  SURFACE  FEATURES 


397 


I.  Folded  Mountains. — This  type  is  much  the  most 
important,  comprising,  as  it  does,  the  greatest  chains  and 
ranges  of  the  Old  and  New  Worlds.  The  Alps,  the  Pyrenees, 
the  Carpathians,  the  Himalayas,  are  all  folded  mountains, 
and  the  same  may  be  said  of  the  great  mountain  chains 
which  extend  almost  continuously  along  the  western  borders 
of  North  and  South  America,  and  the  corresponding  chains 
and  ranges  of  Eastern  Asia  and  its  archipelagoes. 

All  these  mountains,  however  much  they  may  differ  in 
configuration,  are  characterised  by  a  well-marked  geological 
structure.  They  are  composed  essentially  of  highly  flexed 
and  folded  strata.  No  doubt  they  show  other  structures — 
besides  being  folded,  the  rocks  are  often  traversed  by  dis- 
locations large  and  small,  and  by  eruptive  masses  of  different 
kinds.  But  it  is  the  folded  character  of  the  strata  which  is 
the  most  essential  and  typical  structure.  Sometimes  the 
folding  is  of  a  simple  enough  type.  Occasionally,  for  example, 
the  rocks  entering  into  the  formation  of  a  mountain  range 
are  arched  up  in  one  single  broad  saddleback  or  anticline 
(see  Fig.  138).  Or  in  place  of  a  great  geanticline  we 


FIG.  138. — SECTION  ACROSS  THE  UINTA  MOUNTAINS — A  BROAD  ANTICLINE 
BROKEN  BY  A  DISLOCATION  OR  FAULT. 

may  have  a  series  of  many  symmetrical  folds — the  rocks 
rising  and  falling,  as  it  were,  in  a  succession  of  uniform 
undulations  (see  Fig.  1 39).  But  usually  the  structure  is  much 


FIG.  139.— SYMMETRICAL  FOLDS  OF  THE  JURA  MOUNTAINS. 

a,  a,  anticlines  ;  s,  s,  synclines. 

more  complex — the   folds  being  no  longer  open   and  sym- 
metrical, but  closely  compressed  and  inclined  at  all  angles, 


398  STRUCTURAL  AND  FIELD  GEOLOGY 

or  even,  in  many  places,  quite  overturned  and  lying  on  their 
sides  (see  Fig.  140).  This  complex  flexing  and  folding  is 
usually,  as  already  indicated,  accompanied  by  great  disloca- 
tions and  displacements  of  the  strata,  and  not  infrequently 
by  the  appearance  of  veins,  dykes,  and  irregular  masses  of 
formerly  molten  matter,  which  may  traverse  the  disturbed 
strata  in  all  directions. 

When  the  present  external  form  or  configuration  of  folded 
mountains  is  compared  with  their  internal  or  geological 
structure,  the  two  are  seldom  found  to  coincide.  The 
longitudinal  ranges  and  intermediate  longitudinal  valleys  of 
a  mountain  chain  do  not  correspond  save  in  a  very  general 
way  with  flexures  or  folds  of  the  strata.  Mountains  do  not 
necessarily  or  even  often  coincide  with  saddle-backed  or 


FIG.  140. — ALPINE  TYPES  OF  UNSYMMETRICAL  FOLDS. 

arched  structures — nor  do  valleys  invariably  or  even  frequently 
correspond  with  trough-shaped  arrangements  of  the  strata. 
Possibly,  when  a  mountain  chain  came  into  existence,  the 
external  form  of  the  region  may  have  been  more  or  less 
clearly  an  expression  of  the  underground  structure.  That 
is  to  say,  the  mountain  ranges  may  have  coincided  with 
anticlines,  or  saddle-backs,  and  the  intervening  hollows  may 
have  corresponded  with  geological  synclines  or  troughs. 
But  so  long  a  time  has  elapsed  since  even  the  youngest 
of  our  mountain  chains  was  upheaved,  that  the  whole  surface 
has  been  greatly  modified.  Everywhere  we  see  evidence  of 
enormous  erosion  and  denudation.  Vast  masses  of  rock 
have  been  gradually  worn  down  and  removed — swept  away 
as  sediment  from  the  heights,  and  distributed  over  the 
adjacent  low  grounds  or  upon  the  floor  of  the  sea.  Thus, 
in  many  places,  the  original  configuration  of  a  chain  has 
been  obliterated — saddle-backed  mountains  have  been  replaced 


STRUCTURE  AND  SURFACE  FEATURES          399 

by  valleys  and  depressions,  while  trough-shaped  strata  have 
been  carved  into  mountain  heights  (see  Fig.  141). 


FIG.  141. — APPALACHIAN  RIDGES  OF  PENNSYLVANIA. 

a,  a,  anticlines  ;  s,  s,  synclines. 

The  forms  which  folded  mountains  ultimately  assume,  under  the 
action  of  denudation,  are  determined  essentially  by  the  character  of  the 
rocks  and  the  mode  of  their  arrangement.  Certain  kinds  of  rock  and 
particular  types  of  structure  are  more  readily  reduced  than  others.  It 
is  the  more  durable  rocks  and  the  stronger  structures,  therefore,  that 
tend  in  the  long-run  to  constitute  the  mountain  ranges  of  a  chain.  In 
the  younger  mountain  chains  of  the  globe  the  remodelling  of  the  surface 
is  only  partially  accomplished.  Consequently,  amongst  these  the  con- 
figuration is  still  in  many  places  an  expression  of  the  underground 
structure.  Individual  ranges  and  intervening  depressions  continue  to 
coincide  more  or  less  closely  with  the  folds  and  displacements  of  the 
strata.  But  with  increasing  age  such  coincidence  becomes  less  and  less 
marked,  until  in  the  oldest  mountain  chains  it  ceases  to  appear.  Amongst 
these  ancient  mountains,  all  weakly  constructed  heights,  such  as  anti- 
clinal ridges,  have  been  reduced,  while  the  more  enduring  rock  arrange- 
ments, such  as  synclines  or  troughs,  no  longer  form  depressions,  but 
have  most  frequently  been  converted  into  elevations  by  the  removal 
of  the  weaker  structures  which  formerly  dominated  them.  The  contrast 
is  illustrated  by  Figures  139  and  141,  the  former  representing  the  structure 
of  a  portion  of  the  Swiss  Jura — a  relatively  young  chain,  while  the  latter 
shows  the  Appalachian  Ridges  of  Pennsylvania — mountains  of  vastly 
greater  antiquity. 

In  bpth  cases  the  strata,  it  will  be  observed,  are  arranged  in  sym- 
metrical folds,  but,  as  already  stated,  the  structure  of  many  mountain 
chains  is  much  more  complicated — the  folds  being  closely  compressed 
and  pushed  over,  so  as  to  lie  on  their  sides.  The  configuration  ultimately 
acquired  by  mountains  of  the  Alpine  type  naturally  differs  from  that 
assumed  by  an  elevated  region  of  symmetrically  folded  strata.  Generally, 
we  find  that  strata  of  variable  character  which  have  been  compressed 
into  a  succession  of  steeply-inclined  folds  tend  in  time  to  assume  the 
aspect  of  escarpment-mountains.  The  crests  of  the  anticlinal  ridges  are 
removed,  but  the  synclinal  troughs  are  not  developed  into  mountains,  as 
in  the  case  of  symmetrically  folded  strata.  It  is  the  outcrops  of  the 
more  durable  rocks  which  determine  the  position  of  the  heights — which, 
as  a  glance  at  Fig.  142  will  show,  are  escarpments.  The  illustration,  it 
need  hardly  be  said,  is  only  a  diagram.  In  point  of  fact,  the  structure  of 
such  folded  mountains  is  infinitely  more  complex,  the  folding  being 
usually  complicated  by  dislocations  and  displacements  of  all  dimensions, 


400  STRUCTURAL  AND  FIELD  GEOLOGY 

and  not  infrequently  by  abundant  intrusions  of  igneous  rock.  But  in 
ancient  mountains  of  this  type  the  forms  worked  out  by  erosion  and 
denudation  are  in  every  case  determined  by  the  character  of  the 
rocks  and  the  mode  of  their  arrangement.  And  as  highly  inclined  folds 
are  the  rule  in  such  mountains,  the  successive  ranges  carved  out  by 
epigene  action  frequently  coincide  with  the  outcrops  of  the  more  durable 
rocks— they  are  essentially  escarpments— the  escarpments  and  their  dip- 
slopes  becoming  more  and  more  pronounced  as  the  axial  planes  of  the 
folds  are  increasingly  inclined  from  the  vertical. 


FIG.  142. — UNSYMMETRICAL  FLEXURES  GIVING  RISE  TO  ESCARPMENT 
MOUNTAINS. 

s,  s,  relatively  "  soft "  rocks ;  h,  h,  relatively  "  hard  "  rocks. 

If  mountains  are  gradually  lowered  by  denudation,  it 
would  seem  to  follow  that  they  must  in  time  be  wholly 
reduced.  Such,  indeed,  has  been  the  fate  of  not  a  few 
mountain  chains  of  great  geological  antiquity.  Frequently 
we  encounter  plateaus  and  plains  which,  notwithstanding 
their  superficial  form,  have  all  the  structural  characteristics  of 
folded  mountains.  Thus,  in  former  ages,  an  extensive  range 
of  mountains  stretched  across  what  are  now  the  low-lying 
plains  of  Belgium.  The  structure  of  that  region  is  highly 
complicated,  the  strata  being  arranged  in  a  succession  of 
closely  compressed,  unsymmetrical  folds,  so  that  younger 
rocks  are  folded  underneath  older  rocks,  while  here  and  there 
the  crust  has  yielded  to  the  lateral  movement  by  fracturing, 
and  vast  masses  of  strata  have  been  thrust  forward  over  the 
surface  of  rocks  much  younger  than  themselves.  Yet  the 
gently  undulating  ground  gives  no  hint  as  to  the  presence  of 
these  buried  mountain  structures.  The  old  mountains  have 
been  gradually  removed  and  cast  into  the  sea.  Should  the 
relative  level  of  land  and  ocean  remain  unchanged  for  a 


STRUCTURE  AND  SURFACE  FEATURES  401 

sufficient  space  of  time,  a  similar  fate  must  overtake  all 
mountains.  But  a  multitude  of  facts  conspire  to  assure  us 
that  this  level  has  no  permanency.  We  know  that  certain 
mountain  chains,  after  experiencing  enormous  denudation, 
have  been  submerged  or  partially  submerged,  and  become 
eventually  buried  in  whole  or  in  part  under  fresh  accumula- 
tions of  sediment,  often  of  great  thickness.  Thereafter, 
crustal  movements  have  again  supervened ;  the  old  mountain- 
land  has  bulged  up  under  the  squeeze,  and  a  new  series  of 
folds  has  been  formed  outside  of  and  flanking  the  older 
series.  With  the  re-elevation  of  the  mountain  area  a  new 
cycle  of  erosion  is  inaugurated,  and  in  time  the  whole  region 
may  again  be  levelled,  submerged,  and  eventually  once  more 
uplifted.  Ranges  which  are  the  result  of  one  earth-movement 
alone  are  termed  monogenetic ;  those  which,  like  the  European 
Alps,  owe  their  origin  to  two  or  more  such  movements  are 
known  as  poly  genetic  chains. 

The  younger  folded  mountains  of  the  globe  are  typically  represented 
in  the  Old  World  by  the  great  east  and  west  ranges  of  the  Alps,  the 
Himalayas,  etc.,  and  in  the  New  World  by  the  vast  Cordilleras  of  South 
America  and  the  Rocky  Mountains  of  North  America.  These  chains 
usually  consist  of  a  series  of  more  or  less  parallel  ranges,  which  often 
interosculate  or  merge  into  one  another.  Sometimes  they  extend  con- 
tinuously in  approximately  straight  or  gently  curved  lines  ;  in  other  cases, 
a  chain  may  be  more  or  less  strongly  bow-shaped.  Followed  in  the 
direction  of  the  general  axis,  the  mountains  usually  become  progressively 
higher  until,  towards  the  central  portion,  the  greatest  elevations  are 
attained,  after  which  the  heights,  as  a  rule,  gradually  diminish  in  import- 
ance. While  many  mountain  chains  form  a  compact  system  of  ranges 
throughout  their  whole  extent,  others  divide  and  break  up,  as  it  were, 
into  a  series  of  divergent  ranges.  In  all  cases  the  width  of  a  chain  of 
folded  mountains  is  greatly  exceeded  by  its  length. 

All  these  features,  so  characteristic  of  our  younger  chains,  appear 
likewise  to  have  distinguished  the  older  folded  mountains  of  the  globe, 
many  of  which  are  now  sorely  wasted  and  reduced  in  height,  while  others 
have  been  wholly  levelled. 

2.  Dislocation  Mountains. — These  are  so  termed  because 
they  owe  their  origin,  not  so  much  to  folding  as  to  fracturing 
and  displacement  of  the  crust.  They  usually  occur  in  the 
form  of  more  or  less  isolated  heights  or  irregular  shaped 
masses  of  elevated  ground  rising  abruptly  above  adjacent 
lowlands.  Mountains  of  this  type  are  termed  "  Horste  "  by 
German  geologists,  who  cite  the  Harz  Mountains  as  a 

2  C 


402  STRUCTURAL  AND  FIELD  GEOLOGY 

prominent  example.  They  are  usually  composed  of  very  old 
rocks,  and  are  severed  by  vertical  dislocations  from  the  low 
tracts  that  surround  them.  If  the  student  will  imagine  a 
broad  and  lofty  plateau  to  be  cracked  across  in  different 
directions  by  profound  fractures,  and  the  dislocated  plateau 
to  settle  down  irregularly,  he  will  have  some  notion  of  the 
origin  of  Horste.  Those  segments  of  such  a  fractured  plateau 
which  have  retained  their  original  position  are  Horste,  and  they 
therefore  testify  to  a  former  higher  crustal  level  (see  p.  179). 

Occasionally,  dislocation  mountains  occur  as  a  series  of 
parallel  ranges,  separated  the  one  from  the  other  by  large 
vertical  dislocations  or  normal  faults.  The  ranges  of  the  Great 
Basin,  which  extend  north  and  south  between  the  Sierra 
Nevada  and  the  Wasatch  Mountains,  are  of  this  type,  and  of 
similar  origin  are  the  Vosges  and  the  Black  Forest.  The 


BLACK  FOKBST 


FIG.  143. — SECTION  ACROSS  THE  VOSGES  AND  THE  BLACK  FOREST  (PENCK). 

1.  Granite,  etc. ;  2-7,  Mesozoic  rocks  ;  8-9,  Tertiary  and  later  beds. 

escarpments  of  these  two  mountain  ranges  face  each  other, 
separated  the  one  from  the  other  by  long  parallel  lines  of 
faults,  between  which  the  broad  depression  of  the  Rhine  came 
into  existence  (see  Fig.  143). 

Dislocation  mountains,  like  all  other  elevations,  become 
modified  by  erosion  and  denudation,  and  are  met  with  now 
in  every  stage  of  dissolution.  Speaking  generally,  we  may 
say  that  the  best  preserved  examples  are  of  relatively  recent 
geological  age.  But  some  prominent  Horste  are  of  very 
great  antiquity,  their  persistence  being  due  to  the  simple  fact 
that  they  consist  of  more  durable  rocks  than  the  low-lying 
tracts  above  which  they  rise.  In  every  case,  however,  it  can 
be  shown  that  such  Horste  have  experienced  excessive 
denudation. 

3.  Laccolith  Mountains. — The  leading  characters  of  lacco- 
liths have  been  discussed  in  Chapter  XI 1 1.  Mountains  of  this 


STRUCTURE  AND  SURFACE  FEATURES  403 

type  are  obviously  due  to  the  bulging-up  of  the  crust  over  a 
concealed  mass  of  molten  matter  (Fig.  66,-  p.  191).  Laccolith 
mountains  may  formerly  have  been  conspicuous  in  our  own 
and  other  lands,  where  intrusive  igneous  rocks  abound. 
Many  boss-like  masses  and  thick  lenticular  sheets  of  basalt 
and  other  rocks  form  conspicuous  features  in  the  Scottish 
lowlands  (see  pp.  191-195).  These  at  the  time  of  their 
intrusion  were  covered  more  or  less  deeply  with  sedimentary 
strata  which  they  could  not  pierce,  but  may  well  have  lifted 
up  so  as  to  cause  prominent  dome-shaped  bulgings  at  the 
surface.  But  in  the  case  of  such  ancient  igneous  rocks,  so 
long  a  time  has  elapsed  since  their  intrusion  that  any  super- 
ficial bulging  they  may  have  caused  has  entirely  disappeared. 
Recognisable  laccolith  mountains  are  necessarily  of  recent 
formation. 

Erosion  of  Tectonic  Mountains  and  Resulting  Features. — Some 
tectonic  mountains  date  back  to  a  most  remote  geological  antiquity, 
while  others  are  young — not  a  few  having  come  into  existence  in 
relatively  recent  times.  I  n  the  case  of  the  youngest  mountains  of  this  great 
class,  internal  structure,  as  might  have  been  expected,  not  infrequently 
coincides  more  or  less  closely  with  external  form  or  configuration.  This 
correspondence  is  most  clearly  seen  in  recent  Accumulation  mountains, 
such  as  our  still  active  volcanoes — the  shape  assumed  by  those  mountains 
being  obviously  determined  by  the  arrangement  of  their  constituent 
materials.  Nevertheless,  even  active  volcanoes  do  not  escape  the 
modifying  influence  of  the  various  epigene  agents  of  change,  but  are 
attacked  in  the  same  way  as  mountains  of  every  kind,  old  and  young 
alike.  In  their  case,  however,  the  rate  of  decay  is  usually  exceeded  by 
the  rate  of  growth.  Hence  the  rugged  furrows  and  gorges,  gouged  out 
by  torrents  on  the  flanks  of  a  growing  volcano,  tend  to  be  obliterated 
from  time  to  time  by  the  products  of  successive  eruptions.  But  the  great 
chains  and  ranges  of  Folded  mountains  cannot  thus  repair  the  ravages 
effected  by  epigene  action.  The  growth  of  mountains  of  this  type,  we 
have  every  reason  to  believe,  is  a  very  gradual  and  protracted  process. 
No  sooner,  therefore,  does  upswelling  and  wrinkling  of  the  crust  begin, 
than  the  slowly  ascending  surface  is  attacked  by  all  the  atmospheric 
agents  of  change.  And  so  powerful  and  effective  is  this  action,  that  if 
the  rate  of  crustal  movement  did  not  exceed  the  rate  of  denudation,  no 
mountain  range  could  come  into  existence.  It  would  be  degraded  as 
fast  as  it  grew.  Obviously,  however,  crustal  deformation,  no  matter  how 
gradual  it  may  be,  has  in  many,  if  not  in  all,  cases  exceeded  the  rate  of 
denudation.  Nevertheless,  so  potent  are  the  agents  of  erosion  that  they 
have  succeeded  in  very  greatly  modifying  even  the  youngest  elevations 
of  the  crust. 

Although  no  portion  of  a  growing  mountain  chain  can  escape  this 


404  STRUCTURAL  AND  FIELD  GEOLOGY 

modifying  influence,  it  is  evident  that  the  process  of  degradation  must 
be  carried  on  most  actively  along  lines  of  water-flow.  As  torrents, 
streams,  and  rivers  cut  their  way  down  into  the  massif,  larger  and 
larger  surfaces  of  rock  become  exposed  to  subaerial  action.  The 
shattered  debris,  detached  from  cliff  and  mountain  slope,  slowly  or 
more  rapidly  enters  the  drainage  system,  gradually  becomes  reduced  in 
size,  and  is  eventually  swept  away  in  the  form  of  gravel,  sand,  and  mud> 
beyond  the  limits  of  the  mountain  area.  In  time,  therefore,  profound 
and  broad  valleys  are  ploughed  out,  and  these  continue  to  be  deepened 
and  widened,  as  the  process  of  mountain-making  goes  on.  Thus,  in  the 
great  transverse  valleys  which  radiate  from  the  backbone  of  a  growing 
mountain  chain,  the  rate  of  erosion  keeps  pace  with  or  even  exceeds  the 
rate  of  rock-folding  and  uplift.  New  or  secondary  mountains  gradually 
come  into  existence  along  the  flanks  of  the  primary  elevations — a 
mountain  chain,  in  a  word,  grows  by  the  successive  addition  of  contigu- 
ous parallel  ranges.  But  the  large  transverse  rivers  flowing  out  from 
the  primary  axis  are  not  deflected  by  the  younger  ranges  which  thus 
slowly  rise  across  their  path.  The  rate  of  valley  erosion  exceeds  the 
rate  of  crustal  deformation,  and  thus  mountain  range  after  mountain 
range  is  successively  sawn  across  by  the  primeval  rivers  descending 
from  the  axis  of  the  chain. 

We  may  therefore  conceive  of  the  growth  of  a  polygenetic  mountain 
chain  being  continued  through  a  long  period  of  time — the  gradually 
bulging  and  wrinkling  crust  being  concurrently  worn  and  furrowed  by 
epigene  action.  The  mountain-mass  as  a  whole,  however,  continues  to 
increase  in  elevation,  notwithstanding  the  ravages  of  frost  and  glaciers, 
of  rain  and  torrents,  of  streams  and  rivers.  Only  in  the  valleys  does 
epigene  action  balance  or  exceed  the  elevating  process.  When  at 
last  all  earth-movement  ceases,  the  mountains  are  steadily  reduced  in 
height,  while  the  valleys  continue  to  be  widened  and  deepened,  until 
eventually  the  broad  mountain-land  may  disappear  and  be  replaced  by 
a  gently  undulating  plain — a  plane  of 'erosion. 

Many  such  plains  are  known.  That  they  occupy  the  site  of  vanished 
mountain  chains  is  clearly  indicated  by  their  internal  or  geological 
structure.  Some  of  these  old  plains  of  erosion,  like  that  of  the  Belgian 
coal-fields,  reach  no  great  height  above  the  level  of  the  sea,  while  others 
attain  considerable  elevations,  forming  lofty  plateaus.  A  study  of  such 
plateaus  shows  us  that  a  chain  of  Original  or  Tectonic  mountains,  after 
it  has  experienced  much  denudation — after  it  has  been  reduced  to  its 
base-level  and  replaced  by  a  plain  of  erosion — may  again  be  uplifted. 
The  crust  may  once  more  bulge  up,  and  the  plain  be  gradually  carried 
to  such  a  height  that  it  then  becomes  a  plateau  of  erosion.  Or,  instead 
of  being  thus  elevated,  the  plain  may  become  submerged  for  a  longer 
or  shorter  period  of  time.  During  gradual  and  long-continued  sub- 
mergence, sediment  may  gather  over  the  surface  of  the  drowned  land  to 
such  an  extent  that  the  site  of  the  former  mountain  chain  may  eventually 
be  buried  under  a  thickness  of  many  thousand  feet  of  stratified  materials 
— gravel,  sand,  mud,  etc.  Subsequently,  the  movement  of  depression 


STRUCTURE  AND  SURFACE  FEATURES  405 

ceases,  and  may  be  replaced  by  movement  in  the  opposite  direction — 
a  general  bulging-up  or  elevation  of  the  area  may  take  place.  Should 
such  ensue,  then  the  buried  plain  of  erosion  will  again  rise  out  of  the 
sea,  and  may  even  attain  a  height  of  many  thousand  feet  above  that 
level.  In  that  case  we  should  speak  of  the  newly  formed  plateau  as  a 
plateau  of  accumulation.  A  section  across  it  would  show  that  the  upper 
portion  of  the  elevated  area  consisted  of  a  great  thickness  of  approxi- 
mately horizontal  strata  resting  upon  and  concealing  the  old  plain  of 
erosion. 

Although  Tectonic  mountains  tend  to  be  gradually  ground  down  to 
their  base-level,  it  is  seldom  that  the  cycle  of  erosion  is  quite  completed. 
Long  before  the  mountains  have  entirely  vanished,  renewed  crustal 
deformation  may  take  place,  and  the  much-denuded  area  be  either 
re-elevated  or  submerged,  according  as  the  earth-movement  is  up  or 
down.  In  the  former  case  we  get  a  plateau  of  erosion,  the  surface  of 
which  may  be  more  or  less  irregular — ribbed  and  knotted  with  the 
straggling  cores  and  stumps  of  the  ancient  mountains.  In  the  latter 
case,  the  sorely-denuded  mountain-land,  carried  down  below  sea-level, 
becomes  in  time  covered  with  sediments,  underneath  which  the  lower 
lying  parts  of  the  plain  of  erosion  may  eventually  become  very  deeply 
buried.  Should  the  cores  and  stumps  of  the  old  mountains  remain 
above  sea-level  as  islets,  they  will,  of  course,  escape  burial,  only  to  be 
subject,  however,  to  continuous  erosion.  But  should  they  be  submerged, 
then  they  also  will  in  time  become  partially  or  entirely  concealed  under 
gradually  accumulating  sediments.  At  a  later  period,  should  the  sunken 
area  be  re-elevated  to  a  very  considerable  height,  we  shall  have  a  plateau 
of  accumulation,  consisting  of  approximately  horizontal  strata  resting 
upon  the  irregular  surface  of  the  old  plain  of  erosion.  The  horizontal 
strata  will  naturally  attain  their  greatest  thickness  upon  the  lowest-lying 
portions  of  that  plain,  and  will  thin  away  as  they  approach  the  stumps 
of  the  degraded  mountains — the  summits  of  which  may  even  peer  above 
the  surface  of  the  plateau,  as  so  many  islets  in  a  far-stretching  sea. 

2.  SUBSEQUENT  OR  RELICT  MOUNTAINS.— These  have 
not  been  constructed  or  built  as  mountains,  but  are  merely 
remaining  portions  or  fragments  of  a  formerly  more  exten- 
sive elevated  area.  They  have  been  carved  out  of  an  old 
tableland  and  shaped  into  mountains  by  the  gradual  removal 
of  masses  by  which  they  were  at  one  time  surrounded. 

The  form  assumed  by  Relict  mountains  depends  mainly 
upon  the  nature  and  arrangement  of  the  materials  out  of 
which  they  have  been  carved.  A  plateau  of  accumulation,  for 
example,  tends  to  be  cut  up  into  a  series  of  pyramidal  or 
tabular  mountains,  and  crested  or  flat-topped  ridges,  separat- 
ing the  various  valleys  from  each  other.  And  as  the  latter 
are  deepened  and  widened,  the  massive  segments  of  the  old 


406  STRUCTURAL  AND  FIELD  GEOLOGY 

plateau  become  progressively  narrower  and  gradually  reduced 
in  height.  At  a  later  stage  most  of  these  mountainous 
segments  may  have  disappeared,  and  only  a  few  isolated  cones 
and  ridges  or  truncated  pyramids  may  be  left.  Finally,  every 
height  may  be  levelled,  and  the  old  plateau  be  replaced  by 
a  plain  of  erosion. 

In  the  north-west  of  Scotland  we  have  excellent  examples 
of  Relict  mountains  sculptured  out  of  an  ancient  plateau  of 
accumulation.  In  that  region,  certain  old  crystalline  rocks 
(Archaean  gneiss,  etc.)  had  at  a  very  early  geological  period 
been  reduced  to  a  base-level.  The  plain  of  erosion  thus 
formed  was  then  slowly  submerged,  and  became  in  time 
covered  with  a  great  thickness  of  red  sandstones.  Long 
afterwards  the  whole  region  was  re-elevated,  thus  forming  a 
plateau  of  accumulation,  the  upper  portion  of  which  consisted 
of  thick  red  sandstones  resting  on  the  surface  of  a  plain  of 
erosion  composed  of  Archaean  gneiss.  So  prolonged  a  period 
has  elapsed  since  that  epoch  of  elevation,  that  the  red  sand- 
stones have  been  largely  removed,  and  much  of  the  old  plain 
of  erosion  has  been  re-exposed.  Very  considerable  masses 
of  the  overlying  red  sandstones,  however,  still  remain,  forming 
isolated  cone-like  or  pyramidal  Relict  mountains,  such  as 
Canisp,  Soulvein,  Stackpolly,  and  Coulmore,  or  more  closely 
associated  aggregates  of  similar  shaped  heights,  such  as  the 
Torridon  Mountains. 

Having  glanced  at  the  general  character  of  Relict  moun- 
tains which  have  been  carved  out  of  a  plateau  of  accumulation 
— that  is,  out  of  an  extensive  elevated  mass  of  horizontal  or 
approximately  horizontal  strata— we  may  now  shortly  consider 
the  character  of  the  mountains  which  are  chiselled  out  of  a 
plateau  of  erosion.  The  relatively  level  surface  of  such  a 
plateau  is  the  result  not  of  sedimentation  but  of  denudation. 
A  plateau  of  erosion  may  consist  of  many  different  kinds  of 
rock,  arranged  in  almost  any  way.  In  not  a  few  cases,  such 
plateaus  represent  the  sites  of  vanished  chains  of  Tectonic 
mountains.  Externally  they  have  a  plain-like  surface,  inter- 
nally they  frequently  show  all  the  confused  and  complicated 
structures  which  are  characteristic  of  true  mountains  of 
upheaval.  Plateaus  of  this  kind  are  well  represented  in 
Europe.  The  Highlands  and  Southern  Uplands  of  Scotland, 


STRUCTURE  AND  SURFACE  FEATURES  407 

the  Rhenish  Schiefergebirge,  the  Scandinavian  Mountains  are 
all  examples  of  highly  denuded  plateaus  of  erosion. 

At  a  very  early  geological  period,  lofty  ranges  of  Tectonic 
mountains  extended  over  what  are  now  our  Northern  High- 
lands and  Southern  Uplands.  During  prolonged  ages  those 
ancient  Caledonian  ranges  were  subject  to  erosion,  until 
eventually  they  were  largely  reduced  to  their  base-level — only 
a  few  sorely  wasted  stumps  and  cores  projecting  above  a 
gently  undulating  plain  of  erosion.  Subsequently  depression 
ensued,  and  the  plain  of  erosion  became,  over  considerable 
areas,  more  or  less  deeply  buried  under  sedimentary  deposits. 
To  trace  the  geological  history  in  detail  is  here  impossible — 
it  is  too  long  a  tale  to  tell — and  we  need  do  no  more  than 
realise  the  fact  that  eventually  all  the  depressed  areas  were 
again  re-elevated  en  masse. 

The  Highlands  and  Southern  Uplands  then  appeared  as 
plateaus.  Their  configuration  was  upon  the  whole  plain-like, 
the  peripheral  areas  being  to  some  extent  occupied  with 
approximately  horizontal  sedimentary  strata,  resting  upon 
and  concealing  the  old  plain  of  erosion.  In  the  central  and 
more  elevated  portions  of  the  plateaus  that  old  plain  formed 
the  surface,  and  appears  to  have  been  here  and  there 
diversified  by  more  or  less  abrupt  heights — the  worn  and 
abraded  torsos  of  the  ancient  Caledonian  Mountains. 

In  the  course  of  long  ages  the  plateaus  in  question  have 
experienced  excessive  denudation.  To  such  a  degree,  indeed, 
have  they  been  trenched  and  furrowed  by  multitudinous  valleys, 
that  they  are  now  hardly  recognisable  as  tablelands ;  their 
original  plain-like  character  is  well-nigh  lost.  They  have 
been  converted  into  rolling  uplands,  into  regular  ranges  or 
irregular  groups  and  masses  of  Relict  mountains,  the  con- 
figuration and  distribution  of  which  have  been  determined 
very  largely  by  the  nature  of  the  constituent  rocks  and  the 
mode  of  their  arrangement. 

Sometimes,  as  throughout  the  larger  portion  of  the 
Highlands  and  Southern  Uplands,  the  Relict  mountains  have 
been  sculptured  out  of  the  highly  folded  rocks  forming  the 
old  plain  of  erosion ;  in  other  places,  they  are  simply  remain- 
ing portions  of  the  younger  rocks  which  overlie  that  plain ; 
while  in  not  a  few  cases,  the  upper  part  of  a  mountain 


408  STRUCTURAL  AND  FIELD  GEOLOGY 

consists  of  the  younger,  and  its  basal  portion  of  the  older, 
rocks,  the  line  separating  the  two  series  representing  the  old 
plain  of  erosion. 

The  forms  assumed  by  Tectonic  (Folded)  mountains,  during  the  stage 
of  early  youth,  are  a  more  or  less  direct  expression  of  their  internal 
structure.  The  ranges  coincide  to  some  extent  with  upward  folds  or 
anticlines,  and  the  intervening  parallel  hollows  with  downward  folds  or 
synclines  (see  Fig.  139,  p.  397).  But  with  increasing  age  this  approxi- 
mate correspondence  between  configuration  and  structure  gradually 
disappears,  until  eventually  every  coincidence  of  the  kind  vanishes. 
Under  the  long-continued  operation  of  the  agents  of  erosion,  the 
mountains  are  completely  remodelled  (see  Fig.  141,  p.  399).  When  a 
mountain  chain  has  passed  the  age  of  maturity,  the  distribution  and 
shapes  of  its  component  heights  are  determined  directly  by  the  character 
of  the  rocks  and  their  geological  structure.  In  this  respect,  therefore, 
highly  denuded  Tectonic  mountains  do  not  differ  from  Relict  mountains 
which  have  been  carved  out  of  an  ancient  plateau  of  erosion.  In  both 
cases  it  is  the  character  or  nature  of  the  rocks  and  the  mode  of  their 
arrangement  which  determine  the  position  of  the  heights  and  their 
general  configuration.  Nevertheless,  we  must  distinguish  between  the 
two  kinds  of  mountains.  A  Tectonic  mountain  chain  remains  original 
throughout  all  stages  of  its  existence  ;  it  is  a  true  Deformation  mountain 
chain  until  it  is  at  last  swept  away,  and  replaced  by  a  plain  of  erosion. 
A  Relict  mountain  has  not  been  built  up,  nor  is  it  the  direct  result  of 
crustal  deformation.  It  owes  its  existence  to  erosion  ; — it  is  a  mountain 
of  circumdenudation.  But  the  very  causes  which  have  determined  its 
existence  must  eventually  work  out  its  destruction. 

We  have  referred  to  the  forms  assumed  by  mountains  which  have 
been  carved  out  of  plateaus  of  accumulation — pyramids,  and  truncated 
pyramids  being  the  typical  shapes  of  such  mountains.  Under  favourable 
conditions  mountains  of  this  kind  often  ascend  in  a  series  of  abrupt 
terraces,  or  corbel  steps.  But  much  depends  on  the  nature  of  the  rocks. 
If  the  strata  be  more  or  less  homogeneous  in  character,  the  step-like 
outlines  are  not  likely  to  be  pronounced.  Instead  of  abrupt  pyramidal 
heights,  we  may  have  smooth,  rounded  hills.  The  character  of  the 
climate  has  also  a  powerful  influence — an  arid  climate  fostering  the 
formation  of  more  or  less  abrupt  pyramidal  mountains  ;  while  under 
moist  conditions  the  configuration  of  the  heights  tends  to  be  smoother 
and  less  abrupt.  Nevertheless,  whether  the  horizontally  bedded  rocks 
be  of  one  kind  or  another,  or  show  alternations  of  many  different  kinds, 
and  whether  the  climate  be  dry  or  humid,  equable  or  the  reverse — 
tropical,  temperate,  or  arctic — the  mountains  and  hills  sculptured  by  the 
action  of  the  epigene  agents  are  of  the  same  type. 

Relict  mountains  derived  from  the  erosion  of  folded  and  contorted 
rocks  have,  as  already  shown,  the  general  aspect  of  highly  denuded 
Tectonic  mountains.  Hence  sometimes  we  find  them  extending  in  the 
direction  of  the  outcrops  of  the  more  durable  rock-masses,  and  then 


STRUCTURE  AND  SURFACE  FEATURES  409 

forming  more  or  less  regular  ranges.  In  other  places,  again,  owing  to 
the  presence  of  confused  and  complicated  structures,  the  heights  may 
exhibit  little  or  no  trace  of  alinement,  although  it  is  obvious  that  in  this 
case,  as  in  the  other,  the  position  of  the  mountains  has  been  determined 
by  the  nature  of  the  rocks  and  their  arrangement. 

Plains  and  plateaus  do  not  necessarily  consist  either  of 
horizontal  or  of  highly  contorted  rocks.  Between  these  two 
extremes  of  rock-structure  there  are  many  gradations.  The 
degree  of  crustal  deformation  varies  indefinitely.  There  are 
wide  regions  throughout  which  the  rocks  show  only  long, 
gentle  undulations — the  inclination  of  the  strata  from  the 
horizontal  not  exceeding  a  few  degrees.  The  Midlands  of 
England,  for  example,  are  composed  of  rocks  which  have 
a  general  dip  at  a  low  angle  towards  the  east.  Throughout 
the  Central  Lowlands  of  Scotland,  from  the  base  of  the 
Grampians  to  the  foot  of  the  Southern  Uplands,  steep  dips 
are  exceptional.  And  the  same  may  be  said  of  many  other 
parts  of  the  world.  Away  from  regions  of  mountain-uplift, 
indeed,  there  are  vast  continental  tracts  throughout  which 
the  strata  show  little  disturbance — the  beds  rising  and  falling 
in  more  or  less  gentle  undulations.  Sometimes  the  undula- 
tions succeed  each  other  somewhat  rapidly;  in  other  places 
the  crust  has  been  bent  up  in  one  broad,  depressed  arch, 
measuring  many  miles  across.  In  highly  denuded  countries 
the  tops  of  the  anticlinal  arches — the  crests  of  the  undulations 
— have  invariably  been  removed,  and  the  truncated  ends  of 
the  beds  exposed.  In  other  words  the  shape  or  form  of 
the  ground  does  not  coincide  with  the  undulations  of  the 
strata,  but  is  the  result  of  erosion. 

The  most  characteristic  type  of  hill  or. mountain  carved 
out  of  rocks  which  have  a  gentle  dip  or  inclination  is  the 
Escarpment,  the  conditions  for  the  formation  of  which  are 
referred  to  in  Chapter  XIX.  This  is  the  type  of  hill  most 
commonly  met  with  in  Central  England.  A  glance  at  any 
geological  map  of  the  country  will  show  that  all  the  prominent 
hills  and  high  grounds  are  developed  along  the  outcrops  of 
the  Jurassic  limestones  and  the  Chalk,  and  thus  have  a 
general  northerly  or  north-easterly  trend.  Proceeding  from 
the  foot  of  the  Malvern  Hills  towards  the  east,  we  first 
traverse  low-lying  plains  of  sandstone  and  argillaceous  strata, 
until  on  the  other  side  of  the  Severn  we  reach  the  Cotswolds, 


410  STRUCTURAL  AND  FIELD  GEOLOGY 

composed  principally  of  limestones,  which,  as  they  dip  gently 
eastwards,  are  succeeded  by  a  series  of  argillaceous  beds, 
forming  again  a  region  of  undulating  plains.  Continuing 
our  traverse  in  the  direction  of  the  dip,  we  eventually 
encounter  another  broad  belt  of  high  ground — the  escarp- 
ment of  the  Chalk.  This  escarpment  is  succeeded  in  its 
turn  by  a  low-lying  region  composed  mostly  of  soft  clay 
strata,  and  other  more  or  less  non-indurated  materials. 

When  the  strata,  instead  of  being  inclined  in  one  direction 
for  long  distances,  are  arranged  in  a  series  of  gentle  folds, 
escarpments  continue  to  present  themselves  wherever  relatively 
hard  beds  crop  out  at  the  surface.  In  this  way  we  not 
infrequently  find  lines  of  escarpment  facing  each  other,  as 
in  the  well-known  case  of  the  North  and  South  Downs  which 
overlook  the  intervening  Weald.  Here  we  have  an  example 


5.  DOWNS,.  ~ 


FIG.  144.—  SECTION  ACROSS  THE  WEALDEN  AREA— A  DENUDED  ANTICLINE. 

of  a  highly  eroded  anticlinal  fold,  which  is  always  a  weak 
structure,  and  readily  reduced  by  epigene  action.  (Fig.  144.) 

Synclinally  arranged  strata  do  not  succumb  so  easily — the 
structure  is  relatively  strong,  and  makes  a  stouter  resistance. 
The  rocks  so  arranged  are  not  degraded  so  rapidly  as  the 
same  rocks  would  be  if  disposed  in  the  form  of  an  anticline. 
In  short,  it  is  with  gently  undulating  strata  as  with  the 
steeper  and  more  abrupt  convolutions  of  Tectonic  mountains 
— anticlines  are  frequently  replaced  by  hollows,  while  synclines 
tend  to  be  developed  into  heights. 

In  fine,  one  may  say  that  however  simple  or  complex  the 
geological  structure  of  a  highly  denuded  region  may  be,  the 
configuration  of  the  ground,  as  worked  out  by  epigene  action, 
has  been  determined  mainly  by  the  character  of  the  rocks 
and  the  manner  of  their  arrangement.  In  a  word,  when  we 
have  deciphered  the  geological  structure  of  a  country,  we 
have  at  the  same  time  discovered  the  origin  of  its  hills  and 
mountain, 


STRUCTURE  AND  SURFACE  FEATURES  411 

2.  PLAINS  AND  PLATEAUS 

In  discussing  the  structure  and  origin  of  mountains,  some 
reference  has  been  made  to  the  formation  of  plains  and 
plateaus.  It  may  be  well,  however,  to  summarise  here  the 
general  characters  of  those  particular  land-forms. 

(a)  Plains. — These  may  be  defined  as  areas  of  approxi- 
mately flat  or  gently  undulating  land.  They  are  usually 
confined  to  lowlands,  but  in  the  case  of  very  extensive  areas 
the  surface  of  a  plain  may  rise  by  almost  imperceptible 
degrees  to  a  height  of  several  thousand  feet.  This,  however, 
is  exceptional,  such  elevated  tracts  being  usually  termed 
plateaus. 

Plains  of  Accumulation. — These  are  built  up  of  horizontal 
strata,  so  that  the  surface  is  a  more  or  less  exact  expression 
of  the  internal  geological  structure.  Plains  of  this  type  have 
been  formed  in  various  ways.  Many  are  of  lacustrine, 
fluviatile,  or  estuarine  origin ;  in  other  words,  they  consist 
of  undisturbed  aqueous  deposits.  Others,  again,  such  as 
many  coastal  plains,  have  been  formed  partly  by  aqueous 
sedimentation,  and  partly  by  wind  blowing  sand  before  it 
from  exposed  beaches.  When  a  plain  occurs  at  or  near  a 
base-level  of  erosion,  rain  and  running  water  have  little 
effect  upon  it — the  process  of  denudation  is  practically  at  a 
standstill.  Under  certain  conditions,  however,  the  surface 
may  be  considerably  modified  by  the  action  of  wind.  For 
example,  deltas  and  coastal  plains  margined  by  the  sea  or 
by  an  extensive  lake,  are  not  infrequently  invaded  by  sand 
dunes.  The  higher  a  plain  rises  above  its  base-level,  the 
more  does  it  become  subject  to  denudation — high-lying  plains 
usually  showing  a  more  irregular  and  undulating  surface 
than  those  occurring  at  lower  levels.  It  must  be  noted, 
however,  that  the  form  of  the  surface  depends  to  a  large 
extent  upon  the  nature  of  the  materials  of  which  the  plain 
is  composed.  Other  things  being  equal,  a  plain  consisting 
chiefly  of  impervious  deposits  is  more  readily  eroded  than 
one  built  up  largely  of  gravel  and  sand  and  other  more  or 
less  porous  accumulations.  As  examples  of  plains  of 
accumulation  may  be  mentioned  the  fluviatile  plains  and 
deltas  of  the  Nile,  the  Danube,  the  Ganges,  the  Amazon, 


412  STRUCTURAL  AND  FIELD  GEOLOGY 

the  Mississippi,  and  other  rivers,  the  grassy  Steppes  of  Russia, 
the  Aralo-Caspian  depression,  the  Tundras  of  Siberia,  the 
Llanos  and  Pampas  of  South  America,  etc. 

Plains  of  Erosion. — Plains  of  this  class  are  distinguished 
by  the  fact  that  the  surface  does  not  necessarily  coincide 
with  the  underground  structure — in  the  great  majority  of 
cases,  indeed,  there  is  no  such  correspondence.  It  is  only 
when  a  plain  has  resulted  from  the  reduction  of  a  series  of 
horizontal  strata  that  external  form  and  internal  structure 
can  agree.  Plains  of  erosion  may  be  said  to  represent  the 
final  stage  of  a  cycle  of  erosion — they  are  the  base-levels  to 
which  old  land-surfaces  have  been  reduced.  Occurring,  as 
they  usually  do,  at  low  levels,  they  are  liable  to  become 
covered  with  alluvial  and  other  deposits,  and  thus  at  the 
surface  to  show  as  plains  of  accumulation.  Occasionally, 
plains  of  erosion  have  been  submerged  and  covered  more  or 
less  deeply  with  marine  deposits.  Consequently,  when  re- 
elevation  supervened,  the  regenerated  lands  presented  the 
appearance  of  plains  of  accumulation.  It  seems  not  unlikely, 
indeed,  that  the  majority  of  the  latter  are  merely  superimposed 
on  pre-existing  plains  of  erosion.  The  broad,  low-lying  tracts 
through  which  the  larger  rivers  of  the  globe  reach  the  sea 
are  probably  in  many  cases  plains  of  erosion  more  or  less 
covered  and  concealed  under  alluvial  deposits. 

Plateaus  or  Tablelands.— The  term  plateau  is  usually 
applied  to  any  flat  land  of  considerable  elevation  which  is 
separated  from  contiguous  lowlands  by  somewhat  steep  slopes. 
It  is  not  always  possible,  however,  to  draw  a  distinction 
between  plains  and  plateaus — for,  after  all,  a  plateau  is  only  an 
elevated  plain.  Standing  at  a  higher  level  than  plains,  plateaus 
are  necessarily  subject  to  more  active  and  intense  erosion, 
and,  according  to  their  age,  are  correspondingly  incised  and 
denuded.  Plateaus  of  all  kinds,  as  we  have  learned,  tend  to 
acquire  a  mountainous  character — to  be  converted,  in  short, 
into  groups  or  ranges  of  relict  mountains. 

Plateaus  of  Accumulation. — These  are  distinguished  by  the 
fact  that  they  are  built  up  of  horizontal  or  approximately 
horizontal  strata ;  their  general  or  average  surface,  therefore, 
corresponds  with  the  geological  structure.  As  examples  of 
plateaus  of  this  type  may  be  cited  the  Plateau  of  the 


STRUCTURE  AND  SURFACE  FEATURES  413 

Colorado,  the  Uplands  of  Abyssinia,  and  the  Deccan  of  India 
— all  more  or  less  highly  denuded  regions. 

Plateaus  of  Erosion. — The  general  surface  of  these 
plateaus  having  been  determined  by  denudation,  it  rarely 
or  never  coincides  with  the  geological  structure.  But  the 
structure  and  origin  of  erosion  plateaus  have  already  been 
sufficiently  discussed. 

3.  VALLEYS 

By  the  term  valley,  we  usually  mean  the  hollow  or 
depression  through  which  a  stream  or  river  flows.  Some 
valleys,  however,  contain  no  streams,  but  are  mere  elongated 
depressions.  With  relatively  few  exceptions,  valleys  are 
either  (a)  the  direct  result  of  erosion  or  (£)  have  been  greatly 
modified  by  it.  If  we  consider  the  latter,  however,  from  the 
point  of  view  of  their  origin,  they  must  be  distinguished  from 
true  erosion  valleys — just  as  tectonic  mountains  must  be 
recognised  as  such,  even  although  they  have  all  been  more 
or  less  modified  by  denudation.  We  can  therefore  group 
valleys  in  two  classes: — (i)  Valleys  which  owe  their  origin 
either  to  hypogene  action  or  to  epigene  action  other  than 
that  of  erosion ;  and  (2)  valleys  which  are  true  hollows  of 
erosion. 

i.  ORIGINAL  OR  TECTONIC  VALLEYS.— These  are  of  two 
kinds — (a)  elongated  hollows  produced  by  the  irregular 
accumulation  or  heaping-up  of  materials  at  the  surface ;  and 
(ft)  depressions  which  are  the  result  of  crustal  deformation. 

(a)  Constructional  Valleys. — This  class  of  valley  is  of 
little  importance.  It  is  represented  in  volcanic  regions  by 
depressions  occurring  in  the  surface  of  various  volcanic 
accumulations,  and  by  the  now  and  again  pronounced 
hollows  that  separate  adjacent  cones,  lava-flows,  or  heaps 
of  ejecta.  Similarly,  the  depressions  lying  between  ranges 
of  dunes  and  moraines  may  be  termed  constructional  valleys. 
In  a  word,  any  hollow  at  the  surface  caused  by  the  irregular 
distribution  of  materials,  whether  by  volcanic  action  or  by 
epigene  action  of  any  kind,  would  come  under  this  head. 

(£)  Deformation  Valleys. — Theoretically,  we  may  group 
these  as  (i)  Dislocation  Valleys,  and  (2)  Synclinal  Valleys. 
Not  infrequently,  however,  a  deformation  valley  has  been 


414  STRUCTURAL  AND  FIELD  GEOLOGY 

determined  partly  by  fracture  and  partly  by  flexure,  such  as 
the  valley  of  the  Jordan.  Dislocation  valleys  may  extend 
for  long  distances  between  parallel  faults,  or  they  may  follow 
the  line  of  one  great  dislocation  alone.  Such  valleys  are 
approximately  straight  or  gently  sinuous,  and  are  of  not 
infrequent  occurrence.  Glen  App  in  Ayrshire  and  the  great 
hollow  traversed  by  the  Caledonian  Canal  are  examples  in 
this  country.  Another  good  example  is  the  valley  of  the 
Rhine  between  the  Vosges  and  the  Black  Forest.  Synclinal 
valleys  are  best  developed  in  regions  of  recently  uplifted 
tectonic  mountains,  the  surface  features  of  which  not  in- 
frequently coincide  more  or  less  closely  with  the  underground 
rock-structure.  Such  valleys  naturally  trend  in  the  same 
general  direction  as  the  mountains  amongst  which  they 
occur. 

Tectonic  valleys  are,  of  course,  liable  to  modification  by 
erosion.  Dry  valleys,  whatsoever  their  origin  may  have 
been,  may  remain  for  long  periods  comparatively  unchanged. 
It  is  true  that  in  desert  regions  such  valleys  are  subject  to 
the  action  of  the  wind,  which  widens  and  sometimes  deepens 
them,  but  wind-eroded  valleys  are  exceptional.  Wherever 
rain  falls  and  water  flows,  however,  we  look  for  evidence  of 
erosion.  Under  ordinary  conditions  even  the  most  recently 
formed  dislocation  and  synclinal  valleys  have  become  modified. 
And  in  regions  exposed  for  a  prolonged  period  of  time  to 
denudation,  such  valleys  as  coincide  with  dislocations  are 
obviously  wholly  the  work  of  erosion.  They  have  been 
worked  out  along  lines  of  weakness,  which  affect  a  great 
thickness  of  rocks.  However  much,  therefore,  a  land -surface 
may  be  lowered  by  denudation,  the  same  faults  will  continue 
to  guide  the  agents  of  erosion.  The  land  may  have  been 
planed  down  again  and  again  to  base-level — it  may  have 
experienced  more  than  one  cycle  of  erosion — but  with  each 
re-elevation,  valleys  of  erosion  tend  to  reappear  along  the 
same  lines  of  weakness.  Synclinal  valleys,  on  the  other 
hand,  are  much  less  persistent.  When  we  find  a  river 
following  a  synclinal  hollow,  we  may  usually  feel  assured 
that  the  hollow  is  of  relatively  recent  geological  age.  For 
the  synclinal  structure  is  more  durable — less  readily  reduced 
than  the  anticlinal  folds  on  either  side.  The  latter  are  prone 


STRUCTURE  AND  SURFACE  FEATURES  415 

to  collapse,  and  thus  in  time  the  lines  of  drainage  become 
modified.  A  river  which  at  first  followed  a  synclinal  trough 
tends  gradually  to  shift  its  course  as  the  contiguous  anticlines 
are  reduced,  and  ere  long  the  syncline  is  abandoned  in  whole 
or  in  part.  Even  amongst  folded  mountains  of  relatively 
recent  age,  the  longitudinal  or  strike-valleys  often  do  not 
coincide  with  synclinal  troughs — they  have  forsaken  these, 
and  now  flow  in  true  valleys  of  erosion. 

2.  SUBSEQUENT  OR  EROSION  -VALLEYS.— No  hard-arid- 
fast  line  can  be  drawn  between  Tectonic  valleys  and  Erosion 
valleys,  for  many  valleys  are  partly  of  original,  partly  of 
subsequent  origin,  as  is  well  seen  in  regions  of  recent 
mountain-uplift.  In  the  vast  majority  of  cases,  however, 
the  valleys  through  which  rivers  run  are  hollows  of  erosion. 
The  main  lines  of  drainage  have  dbubtless  been  determined 
by  the  original  inclination  of  the  surface — but  the  actual 
formation  of  the  valleys  themselves  is  the  result  of  epigene 
action  alone.  Let  us  picture  to  ourselves  some  extensive 
land-area  just  newly  raised  above  the  level  of  the  sea.  We 
shall  suppose  that  the  surface  is  very  gently  undulating, 
and  that  it  rises  gradually  from  the  sea-coast,  and  culminates 
in  a  more  or  less  abrupt  tract  of  high  ground  which  repre- 
sents, let  us  say,  the  cores  or  stumps  of  some  ancient  reduced 
mountain  chain — mere  torso-mountains  overlooking  a  broad 
tableland  that  sinks  gradually  seawards.  It  is  obvious  that 
the  new-born  rivers  would  necessarily  follow  the  slope  of  the 
ground ;  their  direction  would  be  determined  by  the  con- 
figuration of  the  surface.  At  first  these  primeval  rivers  might 
have  few,  if  any,  tributaries.  As  time  went  on,  however, 
many  lateral  brooks  and  streams  would  come  into  existence. 
The  land,  we  shall  suppose,  consists  of  many  different  kinds 
of  rocks  arranged  in  many  different  ways.  Consequently 
these  would  yield  very  unequally — gradually  relict  hills  would 
come  into  existence,  owing  to  the  reduction  of  the  less 
resistant  rocks  and  rock-structures  in  their  vicinity.  In  a 
word,  the  undulating  land  would  tend  in  time  to  show  a 
more  diversified  surface — heights  and  hollows  would  become 
more  pronounced.  Meanwhile,  the  main  rivers  have  been 
continually  widening  and  deepening  their  courses.  Where 
they  traverse  relatively  hard  rocks,  the  valleys  are  narrow, 


416  STRUCTURAL  AND  FIELD  GEOLOGY 

forming,  it  may  be,  ravines  and  gorges ;  where  only  soft 
rocks  and  yielding  structures  have  been  encountered,  the 
valleys  are  relatively  wide.  In  short,  the  rate  of  erosion 
will  vary  in  the  valleys,  just  as  it  does  over  the  surface  of 
the  land  generally.  Thus  it  will  come  to  pass  that  the 
ravines  and  gorges  of  the  rivers  will  mark  the  outcrops  of 
those  hard  rocks  which,  outside  of  the  valleys,  form  hills, 
ridges,  or  escarpments,  while  the  more  open  reaches  of  the 
valleys  will  coincide  with  the  outcrops  of  the  relatively  soft 
rocks,  which  throughout  the  region  have  determined  the 
position  of  the  lower  grounds.  The  gradual  development 
of  surface  features  implies,  of  course,  the  growth  of  a  secondary 
drainage  system.  Here  and  there  the  trunk  rivers  are  joined 
by  tributaries,  formed  by  surface-waters  making  their  way  down 
the  slopes  of  the  land  and  converging  in  those  depressions  that 
open  directly  upon  the  wider  reaches  of  the  river- valleys.  As 
the  main  rivers  continue  to  deepen  and  widen  their  valleys, 
their  tributaries  will  be  correspondingly  active,  and  still 
younger  brooks  will  begin  to  appear  further  inland,  as  the 
lateral  streams  cut  their  way  back  into  the  heart  of  the 
country.  Eventually,  when  the  whole  drainage  system  has 
reached  maturity,  the  catchment  area  of  each  large  river 
will  show  a  more  or  less  complex  network  of  tributaries  large 
and  small.  The  whole  surface  of  the  tableland  will  now  be 
broken  up  to  such  an  extent  that  it  may  be  hard  to  realise 
its  primeval  configuration.  Nevertheless,  the  general  inclina- 
tion of  the  original  surface  will  be  indicated  by  the  trend  of 
the  chief  rivers.  That  trend  being  quite  independent  of 
the  geological  structure,  the  rivers  have  cut  their  courses 
across  soft  and  hard  rocks  alike — they  seem  to  ignore  all 
obstacles,  traversing  hill-ranges  and  escarpments  just  as  if 
they  had  followed  lines  of  gaping  faults  or  fissures.  All 
these  apparent  obstacles  had  no  existence,  however,  when 
the  rivers  began  to  flow.  They  have  been  slowly  developed 
during  the  gradual  denudation  or  lowering  of  the  surface, 
and  while  the  intersecting  valleys  were  at  the  same  time 
being  deepened  and  widened.  In  a  word,  the  formation  of 
river-gorges,  hill-ranges,  and  escarpments  has  proceeded 
contemporaneously.  The  direction  of  the  main  lines  of 
drainage  has  thus  been  determined  by  the  original  slope 


STRUCTURE  AND  SURFACE  FEATURES          417 

of  the  land,  while  the  subsequent  erosion  of  that  surface 
slope,  guided  and  influenced  by  the  varying  character  and 
structure  of  the  rocks,  has  determined  the  lines  followed 
by  tributary  streams  and  brooks. 

A  typical  river  shows  an  upper  or  torrent-track,  a  middle 
or  valley-track,  and  a  lower  or  plain-track.  In  the  torrent- 
track,  erosion  is  at  a  maximum  and  deposition  of  sediment  at 
a  minimum ;  in  the  valley-track,  erosion  does  not  proceed  so 
rapidly,  while  here  and  there  considerable  deposition  may 
take  place ;  in  the  plain-track,  erosion  practically  ceases  and 
deposition  is  at  a  maximum.  The  plain-track  may  therefore 
be  looked  upon  as  the  base-level  to  which  every  river  strives 
to  reduce  its  bed.  As  erosion  proceeds,  the  plain-track 
gradually  extends  inland  so  as  to  gain  upon  the  valley-track. 
The  latter  in  like  manner  is  continually  encroaching  upon  the 
torrent-track,  while  the  torrent-track  in  its  turn  constantly 
eats  into  the  high  ground  where  it  takes  its  rise.  In  the 
earlier  stages  of  valley-formation,  it  is  obvious  that  the 
original  configuration  of  the  surface  and  the  varying  character 
of  the  rocks  and  rock-structures  may  cause  the  appearance  of 
many  cascades,  waterfalls,  and  rapids  in  all  parts  of  a  young 
river's  course.  But  such  obstructions  tend  gradually  to  dis- 
appear as  erosion  proceeds,  hard  rocks  and  resistant  rock- 
structures  are  eventually  reduced,  and  an  equally  graded 
channel  finally  results.  The  stage  of  maturity  has  now  been 
reached — the  valley  showing  a  true  curve  of  erosion — being 
relatively  steep  in  its  upper  course,  but  rapidly  flattening  out 
as  it  descends  to  the  base-level.  Hence  in  all  regions  which 
have  been  exposed  to  the  action  of  subaerial  erosion  for  a 
prolonged  period  of  time,  considerable  waterfalls  ought  not 
to  occur.  If  they  should  appear  in  a  long-established  hydro- 
graphic  system,  we  may  suspect  that  the  drainage-system 
after  having  attained  maturity  has  subsequently  been  inter- 
fered with.  Waterfalls  cannot  be  of  any  great  age.  Sooner 
or  later  they  must  be  cut  back  and  replaced  by  ravines  or 
gorges.  Their  presence,  therefore,  shows  either  that  the 
valleys  in  which  they  occur  are  throughout  of  recent  age,  and 
that  the  rivers  have  not  yet  had  time  to  reduce  such 
irregularities,  or  that  the  drainage-system,  if  long-established, 
has  since  been  disturbed  by  some  other  agent  than  running 

2  D 


418  STRUCTURAL  AND  FIELD  GEOLOGY 

water.  In  deformation-mountains  of  recent  age,  we  naturally 
expect  to  meet  with  cascades  and  waterfalls,  for  the  streams 
and  rivers  of  such  a  region  are  relatively  young.  They  have 
only,  as  it  were,  commenced  the  work  of  erosion.  But  plains 
and  plateaus  of  erosion  which  have  existed  for  ages  as  dry 
land,  and  in  which  a  complete  hydrographic  system  has  long 
been  established,  should  show  no  great  waterfalls.  Yet  we 
find  cascades  and  waterfalls  more  or  less  abundantly  developed 
in  all  the  plains  and  plateaus  of  Northern  Europe,  and  the 
corresponding  latitudes  of  North  America ;  and  most  of  these 
lands  are  of  very  great  antiquity,  their  main  lines  of  drainage 
having  been  established  for  a  long  time.  Obviously,  the 
hydrographic  systems  have  been  disturbed,  and  the  disturbing 
element  has  been  glacial  action.  During  the  Ice  Age  the 
long-established  pre-glacial  contours  were  greatly  modified. 
Frequently,  indeed,  the  minor  valleys  in  plateaus  and  plains 
were  obliterated,  while  even  the  main  valleys  were  often 
choked  with  debris.  When  glacial  conditions  passed  away, 
and  streams  and  rivers  again  flowed  over  the  land,  they  could 
not  always  follow  the  old  lines  of  drainage  continuously,  but 
were  again  and  again  compelled  to  leave  those,  and  to  cut  out 
new  courses  in  whole  or  in  part.  Hence  the  frequent  occur- 
rence of  cascades  and  waterfalls  in  formerly  glaciated  lands. 

Another  cause  for  the  existence  of  waterfalls  in  long- 
established  drainage-systems  must  be  sought  for  in  crustal 
disturbances.  In  general,  deformations  of  the  crust  would 
seem  to  have  been  very  gradually  brought  about,  so  gradu- 
ally, indeed,  that  they  have  often  had  little  or  no  influence 
upon  the  courses  of  great  rivers.  Anticlines  slowly  develop- 
ing across  a  river-valley  have  been  sawn  through  by  the  river 
as  fast  as  they  arose.  Dislocations,  in  like  manner,  would 
seem  to  have  been  very  slowly  developed.  Frequently  these 
have  traversed  a  river-valley  without  in  any  way  disturbing 
the  drainage,  the  rate  of  erosion  having  been  equal  to  that 
of  displacement.  On  the  other  hand,  we  know  that  faulting 
or  dislocation  may  sometimes  be  effected  suddenly.  Were  a 
fault  to  be  developed  across  a  river- valley  either  suddenly  or 
at  a  greater  rate  than  the  rate  of  erosion,  and  were  its  down- 
throw to  be  in  the  direction  to  which  the  river  flowed,  a 
waterfall  would  certainly  be  the  result. 


STRUCTURE  AND  SURFACE  FEATURES  419 

4.  BASINS 

We  have  now  considered  the  origin  of  the  more  important 
surface  features — mountains,  plains,  plateaus,  and  valleys — 
and  briefly  indicated  to  what  extent  the  varying  character  of 
rocks  and  rock-structures  has  influenced  their  development. 
There  is  yet  another  interesting  class  of  land-forms  deserving 
of  attention  by  the  student  of  structural  geology.  We  refer 
to  the  larger  and  smaller  depressions  of  the  surface,  the 
majority  of  which  are  now  or  have  formerly  been  occupied  by 
water.  Like  other  superficial  features,  Basins  are  of  various 
origin,  some  being  the  result  of  crustal  deformation,  others 
owing  their  formation  to  epigene  action,  while  yet  others  are 
due  to  both. 

(a)  Tectonic  Basins. — Most  of  the  larger  lakes  and 
many  inland  seas  occupy  basins  which  have  come  into 
existence  during  earth-movements.  In  some  cases  these 
depressions  are  geosynclines — the  result  of  a  local  sagging 
or  subsidence  of  the  crust,  not  necessarily  accompanied  by 
fracture  and  dislocation.  In  other  cases,  subsidence  has  taken 
place  along  lines  of  faulting  and  disturbance.  Some  basins, 
again,  would  seem  to  have  come  into  existence  between 
contiguous  high  grounds  undergoing  elevation.  The  great 
lake-basins  of  Russia  and  North  America  (Onega,  Ladoga, 
Superior,  Huron,  Michigan,  etc.),  and  the  extensive  Aralo- 
Caspian  depression,  with  its  numerous  sheets  of  water  and 
desiccated  basins,  are  essentially  geosynclinal  troughs.  The 
Dead  Sea  and  the  lakes  of  Equatorial  Africa,  on  the  other 
hand,  occupy  depressions  caused  by  fracture  and  displacement. 
It  is  worthy  of  note  that  Tectonic  basins  are  not  confined 
to  any  particular  latitude.  A  considerable  number,  apparently 
the  majority,  occur  in  relatively  dry  and  rainless  regions — 
both  at  low  and  high  levels.  On  the  other  hand,  not  a 
few  are  met  with  in  temperate  and  well-watered  lands,  of 
which  the  large  Russian  and  North  American  lakes  are  the 
most  notable  examples. 

(&)  Volcanic  Basins. — The  most  typical  basins  of  this 
class  mark  the  sites  of  extinct  volcanoes.  Many  lakes,  for 
example,  occupy  the  cup-shaped  depressions  of  volcanic 
cones ;  or  the  deep  concavities  in  the  surface  of  the  land 


420  STRUCTURAL  AND  FIELD  GEOLOGY 

(explosion  craters)  produced  by  paroxysmal  outbursts.  The 
Maars  of  the  Eifel,  and  the  numerous  crater-lakes  of  Auvergne 
and  Central  Italy,  are  well-known  types.  Other  volcanic 
lakes  occupy  what  may  be  termed  barrier  basins,  and  owe 
their  origin  to  the  obstruction  of  the  drainage  by  lava  or 
fragmental  ejecta.  The  Lac  d'Aydat  of  Auvergne,  for 
example,  is  confined  by  a  barrier  of  lava. 

(c)  Dissolution  Basins. — These  may  be  shortly  defined 
as  depressions  of  the  surface  caused  by  the  gradual  removal 
of  underlying  soluble  rock.     They  are  the  result,  in  short,  of 
the  chemical  and  mechanical  action  of  underground  water. 
Such  depressions  are  of  common  occurrence  in  regions  where 
massive  limestones  occupy  the  surface,  and   are  caused  by 
the   falling-in   of  subterranean   galleries,  tunnels,  caves,  etc. 
Owing  to  the  highly  fissured  character  of  the  limestone,  these 
depressions  seldom  contain  lakes.     Now  and  again,  however, 
after  very  heavy  rain  which  the  underground  channels  are 
unable   to   dispose   of  at   once,  temporary   lakes   come  into 
existence.     Even  permanent  lakes  are  occasionally  present  in 
such  regions.     These  may  sometimes  be  the  result  of  crustal 
movements  which  have  brought  the  fissured  bed-rock  under 
the  influence  of  the  subterranean  water-level.     In  most  cases, 
however,  such  lakes  probably  owe  their  origin  to  the  closing 
of  the  underground  outlets  by  the  accumulation  of  red  earth 
and  debris.     Occasionally,1  in  glaciated  limestone  regions,  the 
depressions  have  been  rendered  water-tight  by  the  deposition 
of  morainic  materials.      Dissolution  basins  not  infrequently 
occur   in   places  where   the   bed-rocks,  although   themselves 
of  a  more  or  less  impermeable  character,  are  yet  underlaid 
at  a  greater  or  less  depth  by  soluble  material  such  as  rock-salt 
or  gypsum.     The  gradual  removal  of  these  by  underground 
water   eventually   brings   about   slow   subsidence   or   sudden 
collapse  of  the  surface. 

(d)  Alluvial  Basins. — Owing  to  irregular  accumulation 
of  sediment,  shallow  depressions  are  of  common  occurrence 
in   deltas    and    other    broad    fluviatile    and    estuarine   flats. 
These  during  floods   may  become  lakes,  temporary  or  per- 
manent, as  the  case  may  be.     The  deserted  "  loops  "  of  rivers, 
and  the  pools  and  "  creeks  "  which  occupy  the  deeper  hollows 
of  dried-up  river-courses,  come  under  the  same  head.     Again, 


STRUCTURE  AND  SURFACE  FEATURES  421 

lakes  may  be  formed  in  valleys  by  the  disproportionate 
accumulation  of  sediment  by  a  river  and  its  tributaries.  The 
river,  by  carrying  down  large  quantities  of  material,  may 
gradually  raise  the  surface  of  its  bed  above  that  of  its 
affluents,  in  the  lower  reaches  of  which  barrier-lakes  will 
thus  be  formed.  Or  the  tributary  streams  may  throw  more 
detritus  into  the  main  valley  than  the  river  occupying  the 
latter  can  carry  away.  Barriers  are  thus  produced,  and  large 
valley-lakes  appear  above  the  obstructions,  of  which  some  of 
the  lakes  in  Upper  Engadine  (Silser  See,  Silvaplana  See)  are 
examples. 

(/)  ^Eolian  Basins. — These  are  more  interesting  than 
important,  and  are  naturally  confined  to  relatively  dry 
regions.  Some  owe  their  origin  to  the  erosive  action  of  wind, 
while  others  are  constructional — that  is  to  say,  they  are 
hollows  lying  amongst  wind-blown  accumulations. 

(/)  Rock-fall  Basins. — These  are  caused  by  -landslips, 
etc.,  obstructing  the  drainage,  and  are  usually  of  little 
importance.  As  we  should  expect,  they  are  of  frequent 
occurrence  in  regions  of  recent  mountain-uplift,  where  the 
geological  structures  are  weak  and  liable  to  collapse. 

(g)  Glacial  Basins. — The  basins  included  under  this 
head  are  of  various  origin — some  being  true  hollows  of 
erosion,  others  constructional,  i.e.  due  to  the  unequal  heaping- 
up  of  detritus,  while  many  are  partly  one  and  partly  the 
other.  Hence,  some  lakes  of  glacial  origin  occupy  true  rock- 
basins,  and  others  are  essentially  barrier-basins.  All  the 
large  tectonic  basins,  in  so  far  as  they  are  due  to  crustal 
deformation,  might  be  described  as  rock-basins,  while  certain 
volcanic  basins  would  likewise  come  under  the  same  category. 
But  in  these  cases  the  depressions  are  obviously  related 
to  geological  structures — geosynclines,  dislocations,  crateral 
hollows.  Rock-basins  of  glacial  origin  differ  from  all  others 
in  the  fact  that  they  are  totally  independent  of  geological 
structure  and  the  character  of  the  rocks  themselves.  They  are 
met  with  alike  in  igneous,  metamorphic,  and  derivative  rocks 
— whether  these  be  relatively  "hard"  or  "soft" — and  they 
occur  indifferently  in  regions  of  horizontal,  gently  inclined, 
highly  flexured,  and  contorted  strata.  As  might  have  been 
expected,  both  rock-basins  and  barrier-basins  of  glacial  origin 


422  STRUCTURAL  AND  FIELD  GEOLOGY 

are  confined  to  regions  which  are  proved  by  other  evidence 
to  have  been  formerly  occupied  by  snow-fields  and  glaciers. 
And  those  glaciated  regions  are  pre-eminently  the  lake-lands 
of  the  world.  In  Europe,  for  example,  very  few  lakes  occur 
outside  of  the  glaciated  areas  over  which  ice-sheets  and 
glaciers  formerly  extended — the  more  notable  exceptions 
being  the  volcanic  basins  of  Auvergne,  the  Eifel,  and  Central 
Italy.  In  North  America  the  same  remarkable  distribution 
of  lakes  may  be  seen.  Throughout  the  extensive  regions 
lying  north  of  the  glacial  boundary,  they  are  exceedingly 
numerous,  while  south  of  that  line  they  are  almost  unknown. 
Of  the  few  lakes  which  occur  in  regions  which  have  never 
been  subjected  to  glaciation,  the  more  important  occupy 
tectonic  and  volcanic  basins. 

Unless  they  be  very  capacious  and  extensive,  basins  soon 
become  obliterated.  Erosion  and  sedimentation  are  too 
active  to  permit  of  their  prolonged  duration.  Exceptionally, 
basins  which  occur  in  dry  and  practically  rainless  tracts 
where  erosion  and  deposition  are  at  a  minimum,  may  persist 
for  lengthened  periods  of  time.  The  saline  and  alkaline 
lakes  of  such  regions  are  in  many  cases  visibly  drying  up, 
and  wind-blown  sand  is  encroaching  upon  their  desiccated 
floors.  Again,  tectonic  basins  may  long  outlive  the  land- 
surface  upon  which  they  first  appeared.  Should  the  floor  of 
such  a  basin,  occupied  by  a  great  lake,  continue  to  subside  at 
approximately  the  same  rate  as  it  is  being  filled  up  with 
sediment,  and  the  effluent  river  cannot  in  the  meantime 
succeed  in  draining  the  water  away,  it  is  obvious  that  the 
lake  may  persist  for  a  very  long  time.  A  vast  thickness  of 
sediment  might  come  to  accumulate  upon  its  floor,  although 
the  depth  of  the  lake  might  never  have  exceeded  a  few 
hundred  feet.  The  lake  would  in  such  a  case  form  the  base- 
level  for  all  the  surrounding  region,  the  surface  of  which, 
perhaps  mountainous  to  begin  with,  would  be  gradually 
lowered,  and  might  pass  through  a  complete  cycle  of  erosion 
before  the  lake  ceased  to  exist.  In  a  word,  a  great  lake  or 
inland  sea  may  become  the  burial-place  of  the  high  grounds 
that  drain  towards  it,  for  it  bears  the  same  relation  to  these 
as  an  ocean  to  a  continent.  Relatively  few  lakes,  however, 
occupy  tectonic  basins.  Of  these  the  majority,  as  we  have 


STRUCTURE  AND  SURFACE  FEATURES  423 

seen,  occur  in  dry  regions,  where  river-action  is  at  a 
minimum,  and  where  consequently  the  depressions  caused 
by  crustal  deformation  persist  either  as  dry  or  only  partially 
filled  basins.  The  rate  of  subsidence  has  exceeded  the  rate 
of  sedimentation,  and  as  the  lakes  seldom  or  never  overflow, 
save  only  in  very  wet  years,  their  rims  are  not  cut  down  by 
river-erosion.  The  large  lakes  of  Northern  Europe  and 
North  America  are  also  tectonic  basins,  largely  modified, 
however,  by  glacial  erosion  and  accumulation.  They  prob- 
ably came  into  existence  underneath  the  great  ice-sheets 
which  formerly  covered  those  regions,  and  at  a  time,  there- 
fore, when  ordinary  fluviatile  action  was  largely  in  abey- 
ance. They  could  neither  be  silted  up  by  sedimentation, 
nor  breached  by  the  action  of  outflowing  water,  and  too 
short  a  time  has  elapsed  since  the  glacial  period  to  allow  of 
their  obliteration  by  these  causes.  In  well-watered  regions 
all  depressions  of  the  surface,  whatsoever  their  origin  may 
be,  must  sooner  or  later  disappear — the  beautiful  lakes  of 
our  temperate  lands  and  mountain  areas  are  merely 
evanescent  features. 

5.  COAST-LINES 

The  coast-lines  of  the  globe  are  the  joint-product  of 
hypogene  and  epigene  action.  Their  general  trend  is  mainly 
due  to  crustal  movement,  and  is  naturally  determined  by  the 
position  of  the  continents  in  relation  to  the  great  oceanic 
depression.  The  former  are  nowhere  co-extensive  with  the 
true  continental  plateau,  considerable  areas  of  which  in  many 
parts  of  the  world  are  below  the  sea-level.  When  the 
continental  coast-lines  approach  the  margin  of  that  plateau, 
they  usually  continue  for  long  distances  in  one  direction,  are 
rarely  much  indented,  and  show  few  or  no  fringing  islands. 
Conversely,  when  they  recede  from  the  edge  of  the 
continental  plateau,  their  trend  becomes  irregular,  numerous 
inlets  appear,  and  marginal  islands  often  abound.  A  highly 
indented  coast-line  like  that  of  North-west  Europe,  of  Greece, 
and  other  parts  of  the  Mediterranean  lands,  of  Alaska,  and 
many  other  regions,  is  the  result  of  subsidence — the  inlets 
and  fiords  are  merely  the  submerged  lower  reaches  of  old 


424  STRUCTURAL  AND  FIELD  GEOLOGY 

mountain  valleys,  the  marginal  islets  are  the  higher  portions 
of  otherwise  sunken  tracts. 

On  the  other  hand,  the  forms  assumed  by  coasts  is  the 
result  of  epigene  action,  guided  and  controlled,  so  to  speak, 
by  the  character  of  the  rocks  and  their  geological  structure, 


APPENDICES 


APPENDIX   A 


TABLE   OF    BRITISH    FOSSILIFEROUS   STRATA 


!  Raised      Beaches      and      Estuarine 
Flats ;    Lacustrine    and    Fluviatile 
Deposits;    Bogs  or  Peat-Mosses  ; 
Moraines       and       Fluvio  -  glacial 
Gravels,  etc. 
|Xos^GeCr;andnteFfl± 
' (     glacial  Deposits. 


Pliocene 
Miocene 

Oligocene 
Eocene  . 


'NEWER 


OLDER 


Cretaceous   . 


'UPPER 

MIDDLE 
LOWER 

•UPPER 
LOWER 


rCromer  Forest-bed  Group. 

I  Weybourn  and  Chillesford  Crags. 
'  1  Norwich  Crag. 

[Red  Crag.     ' 

/St  Erth  Beds  ;  Coralline  Crag;  Len- 
'  (     ham  Beds,  etc. 
.    (Wanting  in  Britain.) 

IHamstead  Beds. 
Bembridge  Beds. 
Osborne  or  St  Helen's  Beds. 
Headon  Beds. 
Bovey  Tracey  Beds  and  Leaf  Beds  of 
Mull,  Skye,  and  Antrim. 
/Headon  Hill  Sands;    Barton  Clay; 

•  (     Upper  Bagshot  Sands. 
JBracklesham  Beds  and  Middle  Bag- 

•  ^     shot  Sands. 

Lower  Bagshot  Sands. 

London  Clay  and  Bognor  Beds. 

Oldhaven  Beds. 

Woolwich  and  Reading  Series. 

Thanet  Sands. 

Upper  Chalk. 

Middle  Chalk. 

Lower  Chalk  and  Chalk  Marl. 

Upper  Greensand. 

Gault. 

Lower  Greensand. 


1 

.-[Weald  Clay. 
(Hastings  Sand. 


426 


APPENDICES 


Jurassic 


UPPER  or  PORT- 
LAND OOLITES 

MIDDLE    or    OX- 
FORD OOLITES  , 


LOWER  or  BATH 
OOLITES   . 


.LIAS    . 


Triassic 


Permian 


I 
UPPER   or    KEU- 
PER  . 
MIDDLE 
vLOWERorBtJNTER 


f 


UPPER 
LOWER 

COAL-MEASURES. 

MILLSTONE  GRIT 

CARBONIFEROUS 
LIMESTONE 


Devonian   and   Old   Red    Sand- 


Carboniferous 


stone  . 


Silurian  . 


I  UPPER 
LOWER 
(JJrdoviciari) 

fUPPER 


^Cambrian 
Archaean  and  Pre-Cambrian 


A  MIDDLE 
LOWER 


iPurbeck  Beds. 
Portland  Beds. 
Kimeridge  Clay. 
Coral  Rag. 
Oxford  Clay. 

Cornbrash  and  Forest  Marble. 

Great  or  Bath  Oolite,  with  Stones- 
field  Slate. 

Fuller's  Earth. 

Inferior  Oolite. 

Upper  Lias. 

Marl  stone. 

Lower  Lias. 

Penarth  Beds. 
|  New  Red  Marl. 
'-Lower  Keuper  Sandstone. 

(Wanting  in  Britain.) 

Upper  Mottled  Sandstone. 
1  Pebble  Beds. 
VLower  Mottled  Sandstone. 
J  Red  Sandstones,  Clays,  etc. 
|  Magnesian  Limestone. 
I  Marl  Slate. 

J  Red  Sandstones,  Clays,  Breccias,  and 
\     Conglomerates. 

?Red   Sandstones,   and    Upper    Coal- 
bearing  Series. 
-!  Middle  Coal-bearing  Series. 
I  Lower    Coal-bearing    Series   ("Gan- 
(     ister  Beds  "). 

Thick  Sandstones,  etc. 

Yoredale  Beds. 

Main  or  Scaur  Limestone. 

Lower  Limestone  Shale  (England), 
and  Calciferous-  Sandstones  (Scot- 
land). 

Upper  Devonian  "|  Upper  and  Lower 

Middle         „  Old  Red  Sand- 

( Lower          ,,        J      stone. 
fLudlow  Group. 
-J  Wenlock  Group. 
I  Llandovery  Group. 
|  Bala  and  Caradoc  Group, 
-j  Llandeilo  Group. 
[Arenig  Group. 
f  Tremadoc  Slates. 
^Lingula  Flags. 

Menevian  Group. 

Harlech  and  Llanberis  Group. 
(Torridon  Sandstones,  etc. 
^Lewisian  or  Hebridean  Gneiss. 


APPENDIX  B 

THE   SCALE   OF  HARDNESS 

1.  Talc.  6.  Orthoclase. 

2.  Rock-salt  or  Gypsum.  7.  Quartz. 

3.  Calcite.  8.  Topaz. 

4.  Fluor-spar.  9.  Corundum. 

5.  Apatite.  10.  Diamond. 


APPENDIX  C 

TRUE   AND   APPARENT   DIP 

In  the  late  Professor  A.  H.  Green's  admirable  Physical  Geology  the 
student  will  find  a  full  statement  of  the  methods  employed  for  ascertain- 
ing the  amount  and  direction  of  the  true  dip  from  measurements  of  the 
apparent  dip  exposed  in  two  sections  making  a  large  angle  with  one 
another.  The  student  may  usefully  consult  also  Mr  Penning's  Field 
Geology,  and  the  same  author's  Engineering  Geology,  on  the  same 
subject.  A  little  experience,  however,  will  soon  convince  the  observer 
that  the  accuracy  of  an  isolated  dip  obtained  in  this  way  can  only  be 
relied  upon  for  the  particular  spot  at  which  it  is  taken.  Unless  it  be 
confirmed  by  more  or  less  numerous  observations  in  the  neighbourhood, 
the  precise  direction  and  amount  of  the  dip  calculated  from  two  or 
more  apparent  dips  may  not  have  much  significance.  We  judge  so  from 
the  fact  that  in  regions  where  outcrops  are  abundantly  exposed,  the 
direction  and  angle  of  dip  often  vary  within  short  distances.  Not  much 
reliance,  therefore,  can  be  placed  upon  an  isolated  dip,  no  matter  how 
carefully  its  direction  and  angle  have  been  calculated.  The  probabilities, 
however,  are  that  the  field  geologist  will  seldom  or  never  have  occasion 
to  use  any  of  the  methods  or  formulae  referred  to. 


427 


APPENDIX  D 

THE   SPECIFIC   GRAVITY  OF   ROCKS 

Walker's  Specific  Gravity  Balance  is  largely  used  by  geologists,  the 
results  obtained  from  it  being  sufficiently  accurate  for  all  practical 
purposes.  The  principle  of  this  little  machine  will  be  readily  seen  from 
the  accompanying  illustration. 


"  A  B  is  a  lever  resting  on  knife-edges  at  X,  and  graduated  from  X  to  B  in  inches  and 
tenths  (hundredths  must  be  guessed).  W  is  a  weight  which  can  be  moved  out  or  in  on  A  B 
to  suit  the  size  of  the  specimen  weighed.  S  is  an  upright  with  a  vertical  slot  to  receive 
and  steady  the  lever. 

The  piece  of  rock  or  mineral  to  be  tested  is  suspended  from  the  lever  A  B  by  a  fine  thread, 
and  weighed  in  air.  The  exact  point  of  suspension  is  noted;  suppose  it  to  be  Z,  the  distance 
from  X  is  therefore  X  Z.  The  specimen  is  then  immersed  in  water,  as  shown  in  sketch,  care 
being  taken  to  remove  by  means  of  a  brush  any  air-bubbles  that  may  adhere  to  it,  and 
reweighed  ;  suppose  the  point  of  suspension  now  to  be  Y,  the  distance  from  X  is  therefore 
X  Y. 

Now,  since  as  follows  from  the  properties  of  a  lever,  the  weights  in  air  and  water  are  in 
inverse  proportion  to  the  distances  of  the  respective  points  of  suspension  from  X,  and  sirioo 
the  amount  lost  by  immersion  is  exactly  represented  by  the  difference  between  the  latter,  it  is 
evident  that— 


VA 
XY  — 


the  specific  gravity  of  the  specimen. 


An  example  may  perhaps  show  this  more  clearly  :  —  Suppose  that  in  weighing  in  air  Z,  the 
point  of  suspension  is  10  inches  from  X,  and  that,  on  reweighing  in  water,  Y  is  15  inches  from 
X,  then  by  the  above  formula  — 

"VV  1  ^ 

-^T*        =  A  =  3  =  the  specific  gravity  of  the  specimen. 

X  Y  —  \A         15  ~  10 

The  result  may  be  checked  by  changing  the  points  of  suspension  of  W,  and  then  reweighing 
the  object." 

The   Balance   may  be  obtained  from   the   maker,    Mr   G.  Lowdon, 
Reform  Street,  Dundee. 


428 


APPENDIX  E 


COMPASS   AND   CLINOMETER 

Any  ordinary  pocket-compass,  if  not  too  small,  will  serve  to  take  the 
direction  of  dip.  The  one  in  common  use  by  field  geologists  is  divided 
into  graduated  quadrants.  From  North  and  South  points,  the  figures  in 
each  quadrant  run  up  on  either  side  to  90.  In  taking  an  observation, 
allowance  must  be  made  for  the  declination  or  variation — magnetic  north 
in  these  islands,  at  the  present  time,  being  about  20°  west  of  true  north. 
As  all  maps  are  constructed  with  reference  to  the  true  meridian,  it  is 
necessary  that  the  dip-arrows  we  insert  should  likewise  indicate  true  and 
not  magnetic  directions.  In  recording  an  observation  of  dip,  we  say  that 
the  direction  is  so  many  degrees  west  of  north,  east  of  south,  north  of 
west,  and  so  on,  as  the  case  may  be.  Thus  N.  25°  W.  means  25°  west  of 


north,   and    corresponds    therefore   to   the   intermediate  compass-point 
of  NNW. 

Not  infrequently,  owing  to  the  absence  or  paucity  of  streams,  roads, 
fences,  buildings,  etc.,  the  geologist  may  sometimes  have  difficulty  in 
locating  the  position  upon  the  map  of  some  outcrop  or  other  field  data, 
which  he  desires  to  indicate.  In  order  to  do  so  he  must  of  course  take 
bearings  with  the  compass,  for  which  purpose  such  an  instrument  as  that 
shown  in  the  accompanying  illustration  is  usually  sufficient,  but  if  great 
accuracy  be  demanded  a  prismatic  compass  will  be  necessary.  After 

429 


430  .  APPENDICES 

some  little  practice,  however,  the  observer  will  find  an  ordinary  pocket- 
compass  quite  good  enough  for  all  his  requirements. 

In  the  combined  compass  and  clinometer  here  illustrated  a  pendulum 
hangs  perpendicularly  from  the  central  point  which  carries  the  needle, 
and  indicates  the  number  of  degrees  the  straight-edge  or  base,  attached 
to  the  side,  deviates  from  the  horizontal.  In  the  illustration  the  base 
being  level,  the  pendulum  points  to  zero.  One  advantage  of  this  little 
instrument  is  that  the  straight-edge  can  be  applied  either  to  the  upper 
or  under  side  of  a  bed.  Or  if  we  are  taking  the  angle  of  dip  from  a  little 
distance,  and  holding  the  instrument  between  the  eye  and  the  angle  to  be 
measured,  it  is  obviously  immaterial  whether  the  straight-edge  occupies 
the  position  shown  in  the  illustration  or  is  inverted. 


INDEX 


ABYSSAL  eruptive  rocks,  184 
Accessory   minerals    of    igneous   rocks, 

37,  39 

Accumulation  mountains,  394 

Acid  igneous  rocks,  40  ;  mutual  rela- 
tions of,  45 

Acid  secretions  in  granite,  43 

Actinolite,  16  ;  -rock,  78  ;  -schist,  78, 
89 

Adularia,  II 

./Eolian   basins,  421  ;  rocks,  56  ;   soils, 

391 

African  lakes,  419 

After-action.     See  Pneumatolytic  action 
Agate,  5 

Agglomerate,  volcanic,  53,  197 
Alabaster,  30,  66 
Alaska,  indented  coast  of,  423 
Albite,  12,  13 

Alderley  Edge,  ores  at,  261 
Alkali- felspar  rocks,  41 
Allotriomorphic     (anhedral)     minerals, 

36 
Alluvial   basins,  420  ;  clay,    61  ;    soils, 

349 

Almandine,  24 
Alpine  type  of  folds,  398 
Alps,  143,  397,  40i 
Alteration  of  rocks,  37,  212 
Alteration-pseudomorphs,  38 
Alum  shale,  63  ;  slate,  76 
Amazon,  plains  of  the,  411 
Amber,  101 
Amethystine  quartz,  6 
Amianthus,  17 
Amphibole  group,  16 
431 


Amphibolites,  78  ;  soils  from,  382 

Amygdules,  125 

Analcite,  26 

Andalusite,  25,  216  ;  -slate,  77  ;  -horn- 
fels,  79 

Andes,  143 

Andesine,  12,  13 

Andesite,  48,  86,  87,  89  ;  -tuff,  54 

Anhedral  (allotriomorphic)  minerals,  36 

Anhydrite,  29 

Animal-tracks,  118 

Animals,  geological  action  of,  371 

Anorthite,  12,  13 

Anorthoclase,  II 

Anthracite,  71  ;  -slate,  76 

Anticlinal  double- fold,  139 

Anticlines,  136,  392 

Anticlinorium,  140 

Antrim,  plateau-basalts  of,  211 

Apatite,  30  ;  veins  of,  218,  265 

Aphanite,  86 

Aplite,  42 

Appalachian  Mountains,  399 

Aqueous  rocks,  55 

Aragonite,  29 

Aralo-Caspian  depression,  412,  419 

Archaean  rocks,  227,  305 

Arctic  pl;mt-  and  animal-remains  in 
Pleistocene,  98,  318 

Arenaceous  rocks,  soils  from,  383 

Arenaceous  shale,  82 

Argillaceous  rocks,  76,  82,  93  ;  con- 
cretionary, 124;  soils  from,  384 

Arkose,  61,  373 

Artesian  Wells,  360 

Asbestos,  17 


432 


INDEX 


Ascension  theory  of  ore- formations,  270 

Ash,  53 

Asphalt,  72 

Assimilation  of  rocks  by  granite,   etc., 

188,  194,  217,  222 
Association  of  ores  in  lodes,  249 
Augen-gneiss,  77 
Augite,  1 8  ;  -andesite,  49  ;  -diorite,  48  ; 

-gneiss,  77  ;  -syenite,  46 
Augitite,  52 

Aureole,  metamorphic,  216 
Auriferous  deposits,  233 
Autogenous  veins,  206 
Auvergne,  lakes  of,  420,  422 
Avanturine,  5 

BACK  of  lode,  247 

Balas-ruby,  8 

Ball-and-socket  joints,  149 

Ball-granite,  124 

Banded  lodes,  245 

Barytes,  29 

Basalt,  50,  86,  87,  89  ;  joints  in,  148, 

204  ;  weathering  of,  153  ;  soil  derived 

from,  380;  -pumice,  51  ;  -tuff,  54 
Basaltic  hornblende,  17,  19 
Base  in  igneous  rocks,  35,  36 
Base-level   of  erosion,   404,   405,   407, 

412,  414 

Basic  igneous  rocks,  40,  85,  86 
Basic  secretions  in  granite,  42 
Basin  in  valley  of  Rhine  above  Bingen, 

176 

Basins,  414 
Bastite,  19,  49 
Batholiths,  186  ;  metamorphic  effects  of, 

216 

Bedded  veins,  234,  255 
Bed   or  stratum,  108  ;  lenticular  form 

of,  in 

Bed-rock  soils,  376 
Beds,  extent  and  termination  of,  no 
Belgium,  plain  of  erosion  in,  400,  404 
Bendigo  Goldfield,  255 
Biotite,  20  ;  -andesite,  49  ;  -granite,  42 
Bismuth  ores,  249 
Bitter  spar,  29 
Bituminous  shale,  72 
Bituminous  slate  of  Mansfeld,  263 
Blaafjeld,  Norway,  230 


Blackband  ironstone,  29,  68 

Black-earth  of  Russia,  391 

Black.  Forest,  270,  402 

Blind  coal,  71 

Blocks,  volcanic,  53 

Bloodstone,  $ 

Blown  sand,  58 

Bog  iron-ore,  67,  68 

Bohnerz,  258 

Bombs,  volcanic,  53 

Bornite,  249 

Bosses,  189 

Bostonite,  46 

Boulder-clay,  63,  310,  345,  386 

Branching  lodes,  242 

Breaks  in  succession,  181 

Breccia,     volcanic,     53  ;      scree-,     57  ; 

aqueous,    60;     cataclastic,    81,     170, 

207,  305 

Brecciated  lodes,  246 
Brick-earth  or  -clay,  57,  62 
British  Columbia,  229 
Broken  Hill  silver  lode,  256 
Bronzite,  19 
Brown  coal,  71 
Bunches  of  ore,  259 
Bytownite,  12,  13 

CAIRNGORM,  5 
Caking  coal,  71 
Calcareous  concretions,  1 2 1,  124; 

rocks,  82,  93,  124  ;  soils,  335 
Calcite,  28 

Calc-silicate  hornfels,  77,  79,  217 
Calc-sinter,  65 
Calc-spar.     See  Calcite 
Caledonian  Canal,  414 
Caledonian  Mountains,  407 
California,    sandstone    dykes   of,    21 1  | 

placers  of,  234 
Canary  laurel,  98 
Canisp,  406 
Cannel-coal,  71 
Carbon,  31 

Carbonaceous  limestone,  71  ;  shale,  83 
Carbonates,  28 
Carboniferous  limestone,  115,  147,  179  ; 

system,  ores  of,  232,  259 
Carbonisation  of  fossils,  91 
Carinthia,  ore-formations  of,  236,  260 


INDEX 


433 


Carlsbad  twin,  10 

Carnelian,  c 

Carolina,  South,  coprolites  in,  73 

Carpathians,  265,  397 

Carse-clays,  388 

Cascades,  417 

Cassiterite.     See  Tin  ore 

Casts  (fossils),  91 

Cataclastic  rocks,  80,  170,  207,  225 

Cement-stone,  70 

Centroclinal  fold,  135 

Cerussite,  256 

Chabazite,  26 

Chalcedony,  5 

Chalcopyrite,   27,   230,   231,   249.     See 

Copper  ores 
Chalk,  69 
Chalybite,  29 
Charlestown    (S.  Carolina),   coprolites 

at,  73 

Chemically  formed  rocks,  64 
Cherry  coal,  71 
Chert,  5,  67,  84,  120 
Cheviot  Hills,  granite  of,  189 
Chiastolite,  26,  214  ;  -slate,  77 
Chilled  edge  of  basalt,  51 
China-clay,  26,  62 
Chlorite,  22  ;  -gneiss,  77  ;   -schist,  78, 

88,  382 

Chromite,  8,  229,  230 
Chrysolite,  22 
Chrysoprase,  5 
Chrysotile,  23,  80 
Circumdenudation  mountains,  403 
Clastic  ore-formations,  232 
Clay-ironstone,  29,  68,  120,  232 
Clays,  61,  83,  388 
Clay-slate,  76,  82,  382 
Cleat  of  coal,  146 
Cleavage  of  minerals,  3 
Cleavage.     See  Slaty  cleavage 
Climatic      conditions     deduced     from 

fossils,  97 

Clinometer,  127,  276,  429 
Clinozoisite,  24 
Coal,    71  ;    joints   in,    146 ;    coked  by 

igneous  rock,  149  ;    search  for,  330  ; 

origin  of,  333 
Coast-lines,  423 
Cobalt  ores,  249,  263 


Cocardenerze  (cockade  ores),  247 
Coke,  produced  by  action  of  intrusive 

rock  upon  coal,  149 
Coincident  lodes,  240 
Colorado  Plateau,  176 
Comby  lodes,  242 
Common  hornblende,  17,  19 
Common  wells,  358 
Compass,  127,  276,  429 
Complex  faults,  164 
Complex  lodes,  240 
Composite  dykes,  206 
Comstock  lode,  251 

Concretions    and    concretionary    struc- 
tures, 65,  73,  120,  231 
Conformity,  180 
Conglomerate,  60,  84,  94  ;    auriferous, 

257  ;  schistose,  75  ;  crush-,  170 
Consolidation  of  incoherent  accumula- 
tions, 105 

Constructional  valleys,  413 
Contact  metamorphism,  214,  219 
Contact  ore-formations,  260 

Contemporaneous  erosion,  112 

Contemporaneous  igneous  rocks,  208 

Contemporaneous  ore-formations,  228 

Contemporaneous  veins,  207 

Contents  of  fissure- veins,  244 

Contorted  strata,  141 

Contraction,  a  cause  of  jointing,  150 

Convergent  lodes,  242 

Copper,  229;    ores  of,  249,  251,   261, 
263 

Coprolites,  73 

Coralline  Crag,  coprolites  of,  73 

Cordierite,  25 

Cordilleras  of  South  America,  401 

Cornstone,  70 

Cornwall,  ores  of,  234,  251 

Corroded  minerals  in  igneous  rocks,  35 

Corsite,  48,  124 

Corundum,  9 

Coulmore,  406 

Country-rock,  237 

Crag-and-tail,  314 

Crater  lakes,  419 

Cross-bedding,  115 

Cross-galleries  in  coal-mines,  146 

Cross-joints,  147,  152 

Cross-veins,  243 

2  E 


434 


INDEX 


Crush- breccia,  81,  170.     See  Breccia 
Crustal    movements,     slowly    effected, 

J75>  4°3  J  a  cause  of  jointing,  153  ; 

influence  of,  on  coast-lines,  423 
Cryptocrystalline  structure,  36,  43,  44 
Crystalline  igneous  rocks,  40 
Crystalline  schists,  88 
Crystallites,  34 

Cumberland,  ores  of,  259,  269 
Current-bedding,  115 
Current  marks,  116 
Curvature  of  strata,  127,  133 
Curve  of  erosion,  417 
Cycle  of  erosion,  401,  404,  412,  414, 

422 

DACITE,  49 

Danube,  River,  391,  411 

Dead  Sea,  419 

Deep  leads,  234 

Deflation,  58,  369 

Deformation-mountains,  396  ;   -valleys, 

413 
Deformation  of  crust,    slowly  effected, 

175,  403 

Deltas,  1 1 6,  411,  417 

Dendrites,  123 

Denudation,  239,  250,  309,  391,  398, 
410.  See  also  Erosion 

Depth  of  lodes,  239 

Derbyshire,  lead  ore  of,  269 

Derivative  rocks,  55  ;  soils  from,  383 

Desquamation  of  rock-surfaces,  152, 
369 

Determination  of  rocks  in  the  field,  81 

Devitrification,  35,  36,  45 

Diabase,  50 

Diagonal  bedding,  115 

Diallage,  18 

Dichroite,  25 

Diorite,  48,  86,  89  ;  orbicular,  48,  124  ; 
soils  derived  from,  380 

Dip,  127,  427 

Dip-faults,  158 

Dip-joints,  146 

Dip-slope,  296 

Dirt-bed  of  Portland,  100 

Disease  in  relation  to  geological  con- 
ditions, 366 

Disintegration  of  rocks,  368 


Dislocation-mountains,  401  ;  -valleys, 
413 

Dislocations.     See  Faults 

Disseminations,  261 

Dissolution-basins,  420 

Divergent  lodes,  242 

Dolerite,  50,  89 

Dolomite  (mineral)  29 ;  (rock),  65,  82, 
124 

Downs,  North  and  South,  410 

Downthrows,  155 

Drainage,  natural  underground,  350  • 
sewerage,  364  ;  streams  and  rivers, 
415 

Drift  soils,  376,  385 

Drumlins,  Drums,  312,  317 

Druse  and  drusy  cavities,  125,  246 

Dry  valleys,  414 

Dunderlandstal,  ores  of,  235 

Dunes,  391,  396 

D  unite,  52 

Dura  Den,  263 

Dust,  58,  376,  391 

Dykes,  197,  198,  202,  211,  301  ;  com- 
posite, 206  ;  effect  of,  on  underground 
circulation  of  water,  356 

Dynamo-metamorphism,  223 

EARTHQUAKES  a  cause  of  jointing,  154 

Eclogite,  79 

Economic  geology,  330 

Effusive  eruptive  rocks,  208 

Eifel,  crater-lakes  of  the,  420 

•Elgeolite,  14  ;  -syenite,  46 

Elements,  31 

Elk  Mountains,  laccoliths  of,  191 

Embankments,  foundations  of,  349 

Emery,  9 

Enclosures  in  minerals,  4,  37 

End  of  coal,  146 

Endogenous  veins,  207 

Endomorphs,  6,  37 

Engadine,  lakes  of,  421 

Engineering  and  geological  structure, 
340 

Enstatite,  18 

Eocene,  climate  of,  99  ;  contrast  be- 
tween deposits  of,  in  London  Basin 
and  in  Switzerland,  107 

Epidote  group,  24 


INDEX 


435 


Epigene  action,  55,  212,  368 
Epigenetic  ore-formations,  236 
Equipment  for  field  work,  274 
Erosion,  contemporaneous,  112  ;  curve 
of,   417  ;  cycle   of,    400,    403,    407  ; 
valleys    of,    415.     See   also  Denuda- 
tion 
Eruptive     rocks,     structure     of,    184 ; 

mapping  of,  299 
Erzgebirge,  266 
Escarpments,  296,  399,  409 
Eskers,  315 
Essential  minerals  of  igneous  rocks,  37, 

39 

Euhedral  (idiomorphic)  minerals,  36 
Europe,  indented  coasts  of,  423 
Evolution  of  surf  ace- features,  393 
Excavations,  340 

Exfoliation  of  basalt,  etc.,  151,  153 
Exogenous  veins,  206 
Expansion  a  cause  of  jointing,  152 
Eye-gneiss,  77 

FACE  of  coal,  146 

Faeroe  Islands,  plateau-basalts  of,  211 
Fairy-stones,  122 
False-bedding,  115 
Fan-shaped  structure,  140 
Fault-breccia  (fault-rock),  81,  157 
Faults,  155,  295,  298,  401  ;   origin  of, 
171  ;    influence    of,    on    underground 

circulation   of    water,    356  ;    shifting 

of,  169 

Felsite,  44,  84,  85 
Felspars,  9,  40 
Felspathic  rocks,  84 
Felspathoid  rocks,  51 
Felspathoids,  13,  40 
Ferruginous  concretions,  121 
Fibrolite,  26 
Field  equipment,  374 
Field  geology,  273 
Finland,    evidence    of    assimilation    of 

rock  by  granite  in,  189  ;  ore- bearing 

igneous  rocks  of,  229 
Fiords,  423 
Fireclay,  62 

Fish,  sudden  destruction  of,  263 
Fissure-veins,     237,     244.        See     also 

Lodes 


Fissures  of  contraction,  252 

Flagstone,  60 

Flats,  252,  255,  259 

Flexures,  monoclinal,  134.  See  Curva- 
ture of  strata. 

Flint,  5,  67,  84,  120 

Fluidal  structure,  35 

Fluorite,  27,  249 

Fluviatile  deposits,  318,  411 

Fluvio-glacial  deposits,  315,  389,  390 

Fluxion  foliation,  75 

Fluxion  structure,  35 

Folded  mountains,  397 

Folds,  centroclinal,  135  ;  contorted, 
141  ;  normal  or  symmetrical,  135, 
396,  399 ;  origin  of,  142  ;  quaqua- 
versal,  134;  unsymmetrical,  137, 
398 

Foliation,  74,  217,  219 

Footwall,  237 

Forest-beds  in  peat,  319 

Form  of  ground  and  its  relation  to 
geological  structure,  285,  296 

Forsterite,  22 

Fossiliferous  strata,  table  of,  425 

Fossils,  90  ;  rocks  in  which  they  occur, 
93  ;  chiefly  marine,  95  ;  importance 
of,  *in  geological  investigations,  97  ; 
climatic  conditions  deduced  from, 
77  ?  geographical  conditions  deduced 
from,  100 ;  crustal  movements  de- 
duced from,  102  ;  geological  chrono- 
logy dependent  on,  102 

Foundations  in  relation  to  geological 
structure,  345 

Fracture,  zone  of,  271 

Fragmental  igneous  rocks,  53>  88,  210 

France,  North,  former  genial  winters 
in,  98 

Freestone,  60 

Friction- breccia,  170,  171 

Friction-conglomerate,  81 

Frost,  action  of,  56,  63,  371 

Fuller's  earth,  62 

Fusion  of  rocks  by  granite,  189 

GABBRO,  49,  89 
Galena,  246 

Ganges,  plains  of  the,  406 
Gangue,  244 


436 


INDEX 


Garnet  group,  24 
Garnet-hornfels,  79 
Gash-veins,  259 
Geanticline,  140,  397 
Geodes,  42,  125 

Geological  maps  and  sections,  21 
Geological   structure,  economic  aspects 
°f»  33O,  347  J   surface-features  deter- 
mined by,  393 

Geological  surveying,  273 

Geosyncline,  141,  419 

Geyserite,  6 

Giant  granite,  42 

Glacial  basins,  421 

Glacial  rocks,  62,  310 

Glacial  soils,  385,  388 

Glaciated  lands,  frequency  of  waterfalls 
in,  418 

Glacier  lakes,  former,  316 

Glacio-aqueous  clay,  61 

Glass  in  lavas,  35 

Glen  App,  414 

Gneiss,  77  ;  soil  derived  from,  381 

Gold,  27,  229,  232,  250,  261,  265,  266 

Goroblagodat,  contract  ore  of,  265 

Gossans,  248,  337 

Grain  of  granite,  152 

Granite,  41,  87  ;  batholiths  of,  '186 ; 
joints  in,  147  ;  rift  or  grain  of,  152  ; 
secretions  in,  126  ;  soil  derived  from, 
378  ;  xenoliths  in,  188,  218,  222 

Granite-gneiss,  42,  77,  227  ;  -porphyry, 
42 

Granitite,  42 

Granitoid  structure,  41 

Granophyre,  43 

Granophyric  structure,  42,  50 

Granulite,  78,  382 

Graphic  granite,  42 

Graphite,  31,  72 

Graphite-gneiss,  77 

Great  Oolite,  ores  of,  232 

Greece,  indented  coast  of,  423 

Greenland,  plateau-basalts  of,  211 

Greensand,  61  ;  phosphatic  nodules  of 
Upper,  73  ;  iron  ores  in  Lower,  232 

Greisen,  42 

Greywacke,  61 

Grey- wethers,  378 

Grit,  60 


Groundmass,  35,  36 

Guano,  72 

Gypsum,  29,  66  ;  concretions  of,  123 

HADE  of  faults,  154,  159  ;  of  lodes,  237 

Haematite,  6,  67,  259 

Haloids,  27 

Hammers,  geological,  275 

Hanging-wall,  237 

Haplite,  42 

Harz  Mountains,  401  ;  ores  of,  266 

Haiiyne,  14  ;  -phonolite,  48 

Heave  of  faults,  157,  159 

Heavy  spar,  29 

Heliotrope,  5 

Hemicrystalline  igneous  rocks,  35,  84 

Henry  Mountains,  laccoliths  of,  191 

Highlands  (Scottish),  relict  mountains 

of,  407 

Himalayas,  143,  176,  397,  401 
Holocrystalline  rocks,  34,  36 
Horizontal  or  profile  sections,  325 
Hornblende,  17  ;  -andesite,  49  ;  -basalt, 

51;      -gabbro,     49;      -gneiss,     77; 

-granite,  42  ;  -rock,  78  ;  -schist,   78, 

89 

Hornfels,  77,  79,  217 
Hornstone,  5,  67 
Horst  mountains,  179,  401 
Hungary,  ores  of,  251,  265 
Huron,  Lake,  419 
Hyalite,  6 

Hydro-chemical  metamorphism,  222 
Hydro-haematite,  6 
Hydro-mica-schist,  88 
Hypabyssal  eruptive  rocks,  184 
Hyperthene,  1 8  ;  -andesite,  49  ;  -basalt, 

51  ;  -dolerite,  50 
Hypidiomorphic   (subhedral)    minerals, 

36 

ICELAND,  plateau-basalts  of,  211 
Idiomorphic  (euhedral)  minerals,  36 
Igneous    rocks,    classification    of,    40  ; 
geological    structure    of,    184,    202 ; 
general    character    of,    33 ;    mineral 
ingredients   of,  36  ;   mutual  relations 
of,  45,  48,  51  ;   ores  in,   229  ;    secre- 
tions in,  124  ;  soils  derived  from,  378 
Ilmenite,  6,  229 


INDEX 


437 


Imbricated     structure    of    sedimentary 

rocks,  in,  315 
Impounded  streams,  348 
Impregnations,  252,  260 
Inclination  of  strata,  157 
Inclusions  in  minerals,  4,  37,  43 
Incrustation  of  fossils,  91 
Index-beds,  287 
Injection-plexus,  207 
Inliers,  178 

Insolation,  57,  58,  152,  369 
Interbedded  igneous  rocks,  208 
Intermediate  igneous  rocks,  40 
Intersection  of  faults,  169  ;  of  lodes,  242 
Intervals  indicated  by  planes  of  bedding, 

109 

Intratelluric  stage  of  consolidation,  35 
Intrusive  rocks,  184 
Inversion,  138 
lolite,  25 

Iron-hat,  249,  256.     See  Gossans 
Iron  ores,  229,  230,  231,  232,  235,  236, 

249,  258,  264 
Ironstones,  67 

Irregular  ore-formations,  258 
Islands,  fringing  or  marginal,  424 
Isoclinal  folds,  138 
Italy,  crater  lakes  of,  422 

JACYNTH, 8 

Japan,  265 

Jargoon,  8 

Jasper,  5 

Jasp-opal,  6 

Joints,  144 ;  influence  of  changes  of 
temperature  upon,  145  ;  in  bedded 
rocks,  145  ;  in  igneous  rocks,  147, 
198,  204;  in  schistose  rocks,  150; 
origin  of,  150 

Jordan  Valley,  414 

Jura  Mountains,  397 

KAMES,  316 

Kankar,  122 

Kaolin,  62,  126 

Kaolinite,  26,  62 

Keeweenaw  Point,  copper  of,  262 

Kentallenite,  46 

Kersantite,  48 


Kidney-ore,  6 
Klapperstein,  122 
Knotted  slate,  76 
Kugel-granit,  124 
Kupferschiefer  of  Mansfeld,  263 
Kyanite,  26 

LABRADORITE,  12, 13 

Laccoliths,  191,  195,  402 

Lac  d'Aydat,  420 

La  Celle,  tufa  of,  98 

Lacustrine  deposits,  318 

Ladoga,  Lake,  419 

Lake  Caledonia,  373 

Lake  Superior,  copper,  262 

Lakes,  348,  419 

Lamellated  lodes,  245 

Lamination,  107  ;  diagonal,  115 

Landscape-marble,  123 

Lapilli,  53 

Lateral  secretion  of  ores,  269 

Laterite,  58 

Laurvikite,  46 

Lava,  consolidation  of,  35 

Law  of  mineral  combination  in  igneous 

rocks,  39 
Layers,  108 
Leached  guano,  72 
Lead  ores,  249,  256,  269 
Leaf-by-leaf  injection,  188 
Lee-seite,  314 
Leucite,  13  ;  -basalt,  52,  89  ;  -phonolite, 

48 

Leucitite,  52 
Leucitophyre,  48 
Leucoxene,  7,  25 
Level-course  in  coal  mines,  147 
Level-of-no-strain  in  earth's  crust,  142 
Lherzolite,  52 
Lias,  115,  232 
Liebenstein,  dyke  at,  206 
Lignite,  71 
Limburgite,  52,  89 
Limestones,     69,     82 ;     replaced     by 

haematite,  260 
Limonite,  7,  67,  231 
Liparite,  43 
Lithomarge,  26 
Liver-quartz,  372  ;  -rock,  60 
Llanos  of  S.  America,  412 


438 


INDEX 


Loam,  63,  380,  381,  384,  385,  388,  390, 
391,  392 

Loch  Ness,  326 

Lodes,  237  ;  association  of  ores  in,  249  ; 
branching  and  intersecting,  242  ; 
coincident  and  transverse,  240  ;  con- 
tents of,  244  ;  depth  of,  239  ;  hade  or 
underlie  of,  237  ;  heaving  of,  244  ; 
outcrop  of,  247  ;  simple  and  complex, 
240  ;  structure  of,  245  ;  succession  of 
minerals  in,  249  ;  systems  of,  241  ; 
walls  of,  251  ;  width  and  extent  of, 
238 

Loess,  59,  396  ;  concretions  in,  122 

London-clay,  fossils  of,  99 

Longitudinal  faults,  162 

Lower  Greensand,  ores  of,  232 

Lower  Oolite,  115 

Lydian-stone,  67,  84 

MAARS,  420 

Macroscopic  or  megascopic  characters,  I 

Magma-basalt,  52 

Magma  of  igneous  rocks,  33,  35 

Magmatic-extraction     and     magmatic- 

segregation  ore-formations,  267 
Magnetic  iron-ore,  68 
Magnetic  Pyrite.     See  Pyrrhotite 
Magnetite,  7,  229 
Magnesian  limestone,  65 
Malachite,  261 
Malaysian  tin-fields,  234 
Manganese  concretions,  123  ;  ores,  231, 

236,  256,  261 
Mansfeld  copper-slate,  263 
Maps,  277,  278 
Marble,  78,  382 
Marcasite,  28,  122 
Marl,  70,  384 
Marlekor,  122 

Masses  (ore-formations),  258 
Massive  lodes,  245 
Master-joints,  145 
Matrix  in  lodes,  244 
Measurement    of    thickness   of    strata, 

292 

Mechanically-formed  rocks,  56 
Meerschaum,  23 
Meinkjar,  Norway,  230 
Melanite,  24 


Mendip  Hills,  carboniferous  lime- 
stone of,  115 

Menilite,  6,  121 

Mesozoic  iron-ores,  232 

Metals  or  rock-constituents,  266 

Metamorphic  rocks,  74 ;  soils  derived 
from,  381 

Metamorphism,  213,  214,  220,  300,  303 

Metasomatic  replacement,  259 

Mexico,  gold  and  silver  lodes  of,  265 

Mica,  20  ;  -basalt,  51  ;  -diorite,  48  ; 
dolerite,  50  ;  -norite,  49  ;  -schist,  77, 
381  ;  -syenite,  46  ;  -trap,  46,  48 

Michigan,  Lake,  419 

Microcline,  n 

Microfelsitic  matter,  35,  36 

Microgranite,  42 

Microlites,  34 

Micropegmatitic  structure,  42,  43,  50 

Minerals,  rock-forming,  2  ;  inclusions 
in>  4>  37  5  in  igneous  rocks,  36 ; 
primary  or  original,  37,  38  ;  second- 
ary* 37,  39  5  replacing  organic  struc- 
tures, 92  ;  hardness  of,  427 

Minette;  46 

Mississippi,  plains  of  the,  412 

Moh's  scale  of  hardness,  427 

Molecular  replacement,  92 

Molybdenite,  249 

Monoclinal  flexures  and  faults,  173 

Monoclines,  134 

Monogenetic  mountains,  401 

Monte  Nuovo,  196 

Moraines,  64,  314,  396 

Morion,  5 

Moss-agate,  5 

"  Mother  Lode,"  Sierra  Nevada,  238, 
266 

Mottram  St  Andrews,  ore  at,  261 

Moulds  and  casts  (fossils),  91 

Mountain-cork,  17  ;  -leather,  17 

Mountains,  original  or  tectonic,  394 ; 
subsequent  or  relict,  400 

Muscovite,  21  ;  -granite,  42 

Mylonites,  8 1 

NAKKEBROD,  122 

Napoleonite,  124 

Native  metals  in  igneous  rocks,  229 

Natrolite,  26 


INDEX 


439 


Necks,  196,  301  ;  origin  of,  199 

Nepheline,  14;  -basalt,  51,  89 

Nephelinite,  52 

Neutral  rocks  (igneous),  40 

New  Zealand,  229,  265 

Nickel-iron,  in  igneous  rocks,  229 

Nickel  ores,  249,  263 

Nile  Valley,  391,  411 

Nodules,  1 20 

Nordmarkite,  46 

Norite,  49 

Normal  faults,  157,  171,  295,  304 

Normal  folds,  135 

North    America,    lakes     of,   419,   422, 

423 

North  Downs,  410 
Norway,   ore-formations   of,    229,   235, 

265 

Nosean,  14 
Novaculite,  76 


OBLIQUE  bedding,  115  ;  faults,  165 

Obsidian,  44,  49,  85,  89 

Ochil  Hills,  191 

Oil  shale,  62,  72 

Old  red  sandstone,  147 

Oligoclase,  12,  13 

Olivine,  21  ;  -gabbro,  49  ;  -dolerite,  50  ; 

-norite,  49  ;  -rocks,  52 
Omphacite,  18 
Onega,  Lake,  419 
Onyx,  5 
Oolite,  65,  70,  84  ;  Lower,  of  England, 

Oolitic  iron-ore,  67 

Opal,  5  ;  varieties  of,  6 

Ophitic  structure,  50 

Orbicular  diorite,  48,  124 

Ore-formations,  228  ;  origin  of,  266 ; 
prospecting  for,  335 

Ores,  in  igneous  rocks,  229  ;  in  bedded 
rocks,  231  ;  as  precipitates  from 
aqueous  solutions,  231  ;  as  clastic- 
formations,  232  ;  in  schistose  rocks, 
234,  257  ;  in  lodes,  etc.,  237  ;  quasi- 
bedded,  255 

Organically  derived  rocks,  68 

Original  minerals,  37 

Original  mountains,  394  ;  valleys,  413 


Orthoclase,  lo,  II  ;  -porphyry,  46,  87, 
89 

Orthophyre,  46 

Outcrop  of  strata,  130;  concealed,  131, 
290,  308  ;  width  of,  affected  by  dip, 
r3r>  363  ;  width  of,  affected  by  form 
of  surface,  132  ;  affected  by  faults, 
158  ;  tracing  of,  281  ;  forms  of,  291 

Outliers,  176 

Overfolds,  137 

Overlap,  183,  294 

Overthrusts,  174,  298 

Ovifak,  Greenland,  229 

Oxford  clay,  115 

Oxides,  2  ;  metallic,  in  igneous  rocks,  ^ 
229 

PAMPAS,  412 

Paragenesis  of  ores,  249 

Parallel  roads  of  Glenroy,  316 

Parrot  coal,  71 

Peat,  71,  319 

Pegmatite,  42  ;  -veins,  207 

Pegmatitic  structure,  42 

Pencil-slate,  76 

Perched  blocks,  314 

Peridote,  21 

Peridotites,  52,  89 

Perimorphs,  6,  37 

Perlite,  34,  45 

Perlitic  structure,  34,  45 

Permeation  of  fossils,  92 

Permian  copper  ores,  263 

Peru,  261 

Petrifaction,  92 

Petroleum,  72 

Phacoids,  77,  226 

Phenocrysts,  35,  36 

Phonolite,  47,  86,  87,  89 

Phosphates,  30 

Phosphatic  nodules,  73 

Phosphorite,  31 

Phyla  of  animal  kingdom  as  represented 

by  fossils,  96 
Phyllite,  77 
Picotite,  8 
Picrite,  52 
Pipe-clay,  62 
Pipes  of  ore,  259 
Pisolite,  65 


440 


INDEX 


Pistazite,  24 

Pitchstone,  44,  49,  85,  89 

Placers,  232 

Plagioclase,  10,  12 

Plain-track  of  rivers,  417 

Plains  of  accumulation,  411  ;  of  erosion, 

404,  407,412 
Planes  of  bedding,  etc.,  109 
Plants,  geological  action  of,  371 
Plasma,  5 
Plateaus  of  accumulation,  405,  412  ;  of 

erosion,  404,  413 
Platinum,  229,  232,  233 
Platy  lodes,  245 
Pleistocene   period,  climatic  conditions 

during,  98 
Pleonaste,  8 

Plutonic  metamorphism,  220 
Pneumatolytic  action,  62, 126,  2  (9,  265, 

266,  271,  272 
Pockets  of  ore,  259 
Pollar  willow,  98 
Polygenetic  mountains,  404,  406 
Porphyrite,  49,  86  ;  soils  derived  from, 

380.     See  also  Andesite 
Porphyritic  structure,  36 
Potosi,  lodes  of,  251 
Potstone,  23,  78 
Precipitates,  ores,  231 
Primary  minerals,  37,  38 
Prismatic  joints,  148,  151,  204 
Prospecting  for  ores,  335 
Protogine-gneiss,  77 
Pseudomorphs,  7,  38 
Psilomelane,  9,  232,  256 
Pumice,  35,  45 
Pyrenees,  397 

Pyrite,  27  ;  nodules  of,  122,  229 
Pyroclastic  rocks,  53,  210 
Pyrolusite,  9,  232 
Pyrope,  24 

Pyroxene,  1 6,  17  ;  -syenite,  46 
Pyrrhotite,  28,  231,  249 

QUAQUAVERSAL  folds,  134 

Quartz,  2  ;   varieties  of,  5  ;  weathering 

of,  5  ;    in  crystalline  igneous  rocks, 

39 ;     in     clay,    121  ;    -diorite,    48  ; 

-porphyry,   43,  85,  87,  88,  89,  3795 

-schist,  76  ;  -trachyte,  43 


Quartzite,  76,  84,  382 
Quasi- bedded  ores,  255 
Quicksilver  formations,  250,  266 

RADIOLARIAN  chert,  67 

Rain,   action   of,    370;     -prints,    118 ; 

-wash,  57 

Rainless  regions,  basins  in,  419,  422 
Raised  beaches,  317 
Rand  gold-field,  261 
Rapids,  417 
Rattle-stones,  122 
Recumbent  folds,  1 39 
Reefs,  248,  255 

Regional  metamorphism,  220,  303 
Relict  mountains,  405,  415 
Re-opening  of  lodes,  247 
Reservoirs,  348 
Resorption  of  minerals,  35 
Reversed  faults,  170,  173,  298,  304 
Rhenish  Schiefergebirge,  407 
Rhine  Valley,  176,  391,  402,  414 
jlhyolite,  43,  85,  87,  89,  126,  379 ;  -tuff, 

54 

Ridge-faults,  167 

Riesengebirge,  236 

Rift  or  grain  of  rocks,  152 

Rill-marks,  117 

Ring-ores,  247 

Rio  Tinto  iron  ore,  232 

Ripple-marks,  116 

Rivers,  action  of,  415  ;  as  sources  of 
water-supply,  349  ;  older  than  hill- 
ranges  traversed  by  them,  176,  416 

Roches  moutonnees,  313 

Rock-beds,  formation  of,  104 

Rock-crystal,  5 

Rock,  definition  of,  32 

Rock-fall  basins,  421 

Rock-flour,  311 

Rock-forming  minerals,  I 

Rock-guano,  72 

Rock-phosphate,  73 

Rock-rubble,  57,  63 

Rock-salt,  27,  66 

Rocks,  32 ;  crystalline  igneous,  33 ; 
fragmental  igneous,  53  ;  derivative, 
55  ;  metamorphic,  74 ;  determination 
of,  8 1  ;  fossiliferous,  03;  concretionary, 
123 


INDEX 


441 


Rocky  Mountains,  143,  401 

Roman  Camp  Hill,  179 

Roofing-slate,  76 

Rotten-stone,  71 

Rubicelle,  8 

Ruby,  9 ;  spinel-,  8  ;  balas-,  8 

Russia,  undisturbed  palaeozoic  strata  in, 

107  ;  ores  of,  229,  234,  236  ;  lakes  of, 

419 
Rutile,  7 

SADDLE-REEFS,  255 

Sandstone,  60,   84,  93  ;    concretionary, 

123  ;  columnar,  149 
Sandstone-dykes,  2 1 1 
Sand,  volcanic,  53  ;  -dunes,  391,  396 
Sanidine,  II 
Sapphire,  9 
Sarawak,  ores  of,  266 
Sarcen-stones,  378 
Sardonyx,  5 
Satin-spar,  30 
Saussurite,  12 

Scandinavian  Mountains,  407 
Schillerisation,  49 
Schistose  structure,  74 
Schists,    74,   88,   94 ;     fossils    in,    94 ; 

joints  in,  150  ;  ores  in,  234 
Schorl,  25  ;  -granite,  42 
Scoriae,  53 
Scree- breccia,  57 
Seam,  109 

Secondary  enrichment  of  lodes,  249 
Secondary  minerals,  37 
Secondary  mountain-ranges,  401,  404 
Secretionary  ore-formations,  268 
Secretionary  structure,  124 
Secretions  in  granite,  42  ;  in    rhyolite, 

126 
Sections,    horizontal    or    profile,    325  ; 

vertical,  329 
Sedentary  soils,  376 
Sedimentary  ore-formations,  269 
Sedimentary  rocks,  59 
Segregation-veins,  208 
Selenite,  30 
Semi-opal,  6 
Sepiolite,  23 
Septaria,  121 
Serpentine,  23,  79,  213,  221,  382 


Sewage-fields,  dangers  from,  365 

Sgurr  Ruadh,  174 

Shale,  63  ;  alum-,  63  ;  arenaceous,  63  ; 

carbonaceous,  63  ;  oil-,  63 
Shear-structure,  80 
Sheeted  zones,  238 
Sheets,  intrusive.     See  Sills 
Shell-marl,  70 
Shelly  limestone,  70 
Shode-stones,  338 
Shoots,  245 
Siderite,  29,  236 
Sidlaw  Hills,  191 

Signs  used  on  geological  maps,  281 
Silicates,  9 

Siliceous  concretions,  120 
Siliceous  rocks,  82 
Siliceous  sinter,  6,  66 
Sillimanite,  26 
Sills,  192,  300 
Silser  See,  421 
Silvaplana  See,  421 
Silver,  229,  251,  256,  260,  263,  265 
Simple  lodes,  240 
Slates,  76,  224 
Slaty  cleavage,  224,  302 
Slickensided  stones  in  lodes,  245,  251 
Slickensides,  150,  157,  171,  313 
Smaragdite,  17 
Soapstone,  23 
Soda-lime-felspar  rocks,  48 
Sodalite,  14 
Soil,  57,  282,  368 
Soil-cap,  375 

Solenhofen  limestone,  264 
Solfataric  or  after-action,  266,  268,  339. 

See  also  Pneumatolytic  action 
Solutions,  siliceous,  3 
Soulvein,  406 

Southern  Uplands,  Scotland,  407 
South  Downs,  410 
Spathic  iron-ore,  68 
Specific  gravity  of  rocks,  89,  428 
Specular  iron,  6 

Sphaerosiderite,  29  ;  nodules  of,  122 
Sphene,  25 
Spherulite-rock,  45 
Spherulitic  structure,  34,  45 
Spinel,  8 
Spinelloids,  8 


442 


INDEX 


Splint  coal,  71 

Spotted  slate,  76 

Springs,  in  bedded  rocks,  350 ;  in 
igneous  rocks  and  schists,  353  ;  shal- 
low and  deep-seated,  355  >  use  °f>  ^n 
field  geology,  287 

Sprudelstein,  70 

Stackpolly,  406 

Stalactites,  64 

Stalagmites,  64 

Staurolite,  26 

Steatite,  23 

Step-faults,  167 

Steppes,  412  ;  fauna  of,  392 

Stilbite,  26 

Stockworks,  252,  260 

Stone  coal,  71 

Stoss-seite,  314 

Strata,  thinning  and  thickening  of,  112, 
293  ;  grouping  of,  113  ;  contempor- 
aneity of  strongly  contrasted,  114 

Stratification,  107,  109,  115 

Stratified  rocks,  55 

Stratum,  108 

Streaky  structure,  80 

Striae,  glacial,  313 

Strike,  132  ;  -faults,  162  ;  -joints,  146 

Structural  geology,  104 ;  economic 
aspects  of,  330,  347 

Subaerial  rocks,  56 

Subhedral  (hypidiomorphic)  minerals, 
36 

Subsequent  eruptive  rocks,  184 

Subsequent  mountains,  405 

Subsequent  valleys,  415 

Subsoil,  57,  282,  368,  376 

Substitution  pseudomorphs,  38 

Succession  of  minerals  in  lodes,  249 

Sulphates,  29 

Sulphide  ores,  231 

Sulphides,  27 

Sun-cracks,  Il8 

Superficial  accumulations,  307 

Superior,  Lake,  419 

Surface-creep,  129 

Surface-markings,  116 

Surface-wash,  308 

Sweden,  ores  of,  229,  234 

Syenite,  46,  89 

Syenitic  mica-trap,  46 


Symmetrical  folds,  135 
Symmetrical  lodes,  246,  397 
Synclinal  valleys,  352,  413 
Synclines,  136 
Synclinorium,  140 
Syngenetic  ores,  228 
Systems  of  lodes,  241 

TABLELANDS,  412 

Tachylite,  51 

Talc,  23  ;  -schist,  78,  88 

Tar  springs,  72 

Tectonic  basins,  419,  422 

Tectonic  geology,  104 

Tectonic  mountains,  394,  403,  407 

Tectonic  valleys,  413 

Temperature,    underground,    106,    145, 

270 

Terminal  curvature,  129 
Terra  rossa,  58,  385 
Terrestrial  movements,  evidence  of,  102 
Tertiary  deposits,  307 
Thermal  metamorphism,  214,  219 
Thickening  and  thinning  of  strata,  112, 

293 

Throw  of  faults,  156 
Thrust-planes,  174 
Till.     See  Boulder-clay 
Tin-ore  veins,  21 8 
Titanite,  25 
Toadstones,  269 
Tonalite,  48 
Topaz,  249 

Torrent-track  of  rivers,  417 
Torridon  Mountains,  406 
Torso  mountains,  407,  415 
Tourmaline,  25  ;  -granite,  42  ;  -hornfels, 

79 

Trachyte,  47,  86,  87,  89 
Transcurrent  faults,  170 
Transported  soils,  376 
Transvaal,  gold  of,  261 
Transverse  lodes,  240 
Transverse  thrusts,  170,  175 
Travelled  soils,  376 
Travertine,  65 
Trees  in  peat,  319 
Tremolite,  16 
Troctolite,  49 
Trough-faults,  167 


INDEX 


443 


Tufa,  calcareous,  65,  96  ;  of  La  Celle, 

98 

Tuff,  volcanic,  53  ;  concretions  in,  124 
Tundras,  407 

Tunnels,  construction  of,  343 
Turf,  70 
Turgite,  6 

Twinning  of  felspars,  10 
Type-fossils,  103 

UlNTA  Mountains,  397 
Unconformity,  180,  294 
Underground  water,  350,  420 
Underlie,  237 
United  States  (N.  America),  ores  in,  236, 

266 

Unsymmetrical  folds,  137,  398,  400 
Ural  Mountains,  ores  of,  234,  265 
Utah  (N.  America),  265 

VALLEYS,  413 

Valley-track  of  rivers,  417 

Vapour-pores  in  lava,  35 

Vegetable  soil,  376 

Vegetation,  an  index  to  soils,  285 

Veins  (eruptive),  202,  206 

Veinstone  or  veinstuff,  244 

Vertical  sections,  329 

Vesicular  lava,  35,  36 

Victoria  (Australia),  placers  of,  234 

Vitreous  rocks,  34,  85 

Volcanic  agglomerate,  53 

Volcanic  ash,  53 

Volcanic  basins,  419 


Volcanic  bombs,  53 

Volcanic  breccia,  53 

Volcanic  stage  of  consolidation,  36 

Volcanic  tuff,  53,  94 

Volcanoes,  395 

Vosges  Mountains,  402 

Vughs,  246,  247,  256 

WAD,  9,  232 

Walls  of  lodes,  251 

Water  derived   from   plutonic   sources, 

271 

Waterfalls,  417 

Water-level,  underground,  350 
Water-supply,  347 
Wave-marks,  117 
Wealden  area,  232,  410 
Weathering  of  rocks,  56,  58,  151,  153, 

372 

Wells,  358 
Western  Islands,  Scotland,    basalts    of, 

211 

Whet-slate,  76 

White  trap,  194 

Wind,  geological  action  of,  369 

Witwatersrand  gold-field,  261 

Wolframite,  249 

XENOLITHS,  8,  188,  222,  230 

ZEOLITES,  26 

Zinc-ore,  249,  250,  257,  260,  263 

Zircon,  8 

Zoisite,  24 

Zone  of  fracture,  271 


DATE 


OVERDUE. 

— — 


OCT  30 

14  1933       APR 


SEP    151936- 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


